View source for Sleep

{{Short description|Naturally recurring resting state of mind and body}}
{{About|sleep in humans|non-human sleep|Sleep in animals|other uses}}
{{Redirect2|Asleep|Slept|other uses|Asleep (disambiguation)|and|SLEPT analysis}}
{{protection padlock|small=yes}}
{{Use dmy dates|date=May 2024}}
{{Infobox body process
|name              = Sleep
|image             = Domenico Fetti - Sleeping Girl - WGA7863.jpg
|caption           = ''Sleeping Girl'', [[Domenico Fetti]], {{circa|1615}}
|biological system = [[Nervous system]]
}}
'''Sleep''' is a state of reduced mental and physical activity in which [[consciousness]] is altered and certain [[Sensory nervous system|sensory]] activity is inhibited. During sleep, there is a marked decrease in muscle activity and interactions with the surrounding environment. While sleep differs from [[wakefulness]] in terms of the ability to react to [[Stimulus (physiology)|stimuli]], it still involves active [[Human brain|brain]] patterns, making it more reactive than a [[coma]] or [[disorders of consciousness]]<!-- Disorders of consciousness refer to a group of conditions that affect the level or content of consciousness, leading to alterations in wakefulness and awareness. Some examples of disorders of consciousness include coma, vegetative state, minimally conscious state, and locked-in syndrome. -->.<ref>{{Cite web |title=Brain Basics: Understanding Sleep {{!}} National Institute of Neurological Disorders and Stroke |url=https://www.ninds.nih.gov/health-information/public-education/brain-basics/brain-basics-understanding-sleep |access-date=15 February 2023 |website=www.ninds.nih.gov}}</ref>

Sleep occurs in [[sleep cycle|repeating periods]], during which the body alternates between two distinct modes: [[rapid eye movement sleep]] (REM) and [[Non-rapid eye movement sleep|non-REM sleep]]. Although REM stands for "rapid eye movement", this mode of sleep has many other aspects, including virtual [[Rapid eye movement sleep#Muscle|paralysis]] of the body.<ref>{{Cite web |url=https://quantifysleep.com/the-dichotomy-of-sleep-rem-and-non-rem-stages-and-their-impact-on-human-health |title=The Dichotomy Of Sleep: REM And Non-REM Stages, And Their Impact On Human Health |last=Nelson |first=Ryan |date=20 June 2021 |website=Quantify Sleep |access-date=15 July 2023 }}</ref> [[Dream]]s are a succession of images, ideas, emotions, and sensations that usually occur involuntarily in the [[mind]] during certain stages of sleep.

During sleep, most of the [[human body|body's systems]] are in an [[anabolic]] state, helping to restore the [[Immunity (medical)|immune]], [[Nervous system|nervous]], [[Skeleton|skeletal]], and [[Muscular system|muscular]] systems;<ref name="krueger">{{cite journal | vauthors = Krueger JM, Frank MG, Wisor JP, Roy S | title = Sleep function: Toward elucidating an enigma | journal = Sleep Medicine Reviews | volume = 28 | pages = 46–54 | date = August 2016 | pmid = 26447948 | pmc = 4769986 | doi = 10.1016/j.smrv.2015.08.005 |issn=1087-0792}}</ref> these are vital processes that maintain [[Mood (psychology)|mood]], [[memory]], and [[cognitive function]], and play a large role in the function of the [[Endocrine system|endocrine]] and [[immune system]]s.<ref name=":3">{{cite web|date=2006|title=Sleep-wake cycle: its physiology and impact on health|url=https://sleepfoundation.org/sites/default/files/SleepWakeCycle.pdf|publisher=National Sleep Foundation|access-date=24 May 2017|archive-url=https://web.archive.org/web/20170830035939/https://sleepfoundation.org/sites/default/files/SleepWakeCycle.pdf|archive-date=30 August 2017|url-status=live}}</ref> The internal [[circadian clock]] promotes sleep daily at [[night]], when it is dark. The diverse purposes and [[Neuroscience of sleep|mechanisms of sleep]] are the subject of substantial ongoing research.<ref name=":5">{{cite web |url=https://thesciencenetwork.org/programs/waking-up-to-sleep |title=Waking Up To Sleep |access-date=25 January 2008 | vauthors = Bingham R, Terrence S, Siegel J, Dyken ME, Czeisler C |date=February 2007 |format=Several conference videos |publisher=The Science Network |archive-url=https://web.archive.org/web/20110724132410/https://thesciencenetwork.org/programs/waking-up-to-sleep |archive-date=24 July 2011 |url-status=live }}</ref> Sleep is a highly [[Conserved sequence|conserved]] behavior across animal evolution,<ref name=Joiner2016>{{cite journal | vauthors = Joiner WJ | title = Unraveling the Evolutionary Determinants of Sleep | journal = Current Biology | volume = 26 | issue = 20 | pages = R1073–R1087 | date = October 2016 | pmid = 27780049 | pmc = 5120870 | doi = 10.1016/j.cub.2016.08.068 | bibcode = 2016CBio...26R1073J }}</ref> likely going back hundreds of millions of years,<ref name="Keene & Duboue, 2018">{{cite journal | title=The origins and evolution of sleep. | journal=The Journal of Experimental Biology | date=12 June 2018 | volume=221 | issue=11 | doi=10.1242/jeb.159533 | pmid=29895581 | pmc=6515771 | bibcode=2018JExpB.221B9533K | last1=Keene | first1=Alex C. | last2=Duboue | first2=Erik R. }}</ref> and originating as a means for the brain to cleanse itself of waste products.<ref name="ntyimes">{{cite news|last=Konnikova|first=Maria|title=Goodnight. Sleep Clean.|url=https://www.nytimes.com/2014/01/12/opinion/sunday/goodnight-sleep-clean.html|newspaper=[[The New York Times]]|access-date=18 February 2014|date=11 January 2014|quote=She called it the glymphatic system, a nod to its dependence on glial cells}}</ref>  In a major breakthrough, researchers have found that cleansing, including the removal of [[amyloid]], may be a core purpose of sleep.<ref name="science2">{{cite news|last=Coontz|first=Robert|title=Science's Top 10 Breakthroughs of 2013|url=https://www.science.org/content/article/sciences-top-10-breakthroughs-2013|newspaper=[[Science (journal)|Science]]|access-date=12 January 2025|date=19 December 2013}}</ref>

Humans may suffer from various [[sleep disorder]]s, including [[dyssomnia]]s, such as [[insomnia]], [[hypersomnia]], [[narcolepsy]], and [[sleep apnea]]; [[parasomnia]]s, such as [[sleepwalking]] and [[rapid eye movement sleep behavior disorder]]; [[bruxism]]; and [[circadian rhythm sleep disorder]]s. The use of [[artificial light]] has substantially altered humanity's sleep patterns.<ref>{{cite news| vauthors = Randall DK |title=Book excerpt: How the lightbulb disrupted our sleeping patterns and changed the world|url=https://nationalpost.com/opinion/book-excerpt-how-the-lightbulb-disrupted-our-sleeping-patterns-and-changed-the-world|access-date=31 August 2016|work=National Post|date=19 September 2012|quote="... the sudden introduction of bright nights during hours when it should be dark threw a wrench into a finely choreographed system of life.|archive-url=https://archive.today/20190407032206/https://nationalpost.com/opinion/book-excerpt-how-the-lightbulb-disrupted-our-sleeping-patterns-and-changed-the-world|archive-date=7 April 2019|url-status=live}}</ref> Common sources of artificial light include [[Light pollution#Public health impact|outdoor lighting]] and the screens of digital devices such as [[Smartphone|smartphones]] and [[Television set|televisions]], which emit large amounts of [[Blue light spectrum|blue light]], a form of light typically associated with daytime. This disrupts the release of the hormone [[melatonin]] needed to regulate the [[sleep cycle]].<ref>{{Cite web|date=4 November 2020|title=How Blue Light Affects Sleep|url=https://www.sleepfoundation.org/bedroom-environment/blue-light|access-date=18 November 2021|website=Sleep Foundation|language=en}}</ref>

== Physiology ==
{{Main|Neuroscience of sleep}}
The most pronounced physiological changes in sleep occur in the brain.<ref name=MaquetEtAl2006>{{cite book |doi=10.1142/9781860947186_0006 |chapter=Brain Imaging on Passing to Sleep |title=The Physiologic Nature of Sleep |chapter-url=https://books.google.com/books?id=cxFqDQAAQBAJ&pg=PA123 |year=2005 | vauthors = Maquet PA, Sterpenich V, Albouy G, Dang-Vu T, Desseilles M, Boly M, Ruby P, Laureys S, Peigneux P | display-authors = 6 |pages=123–137 |publisher=Published by Imperial College Press and Distributed by World Scientific Publishing Co. |isbn=978-1-86094-557-1 }}</ref> The brain uses significantly less energy during sleep than it does when awake, especially during non-REM sleep. In areas with reduced activity, the brain restores its supply of [[adenosine triphosphate]] (ATP), the molecule used for short-term storage and transport of energy.<ref>[[#Brown|Brown]], pp. 1118–1119: "Compared with wakefulness, sleep reduces brain energy demands, as suggested by the 44% reduction in the cerebral metabolic rate (CMR) of glucose (791) and a 25% reduction in the CMR of O<sub>2</sub> (774) during sleep."</ref> In quiet waking, the brain is responsible for 20% of the body's energy use, thus this reduction has a noticeable effect on overall energy consumption.<ref name=Siegel2008>{{cite journal | vauthors = Siegel JM | title = Do all animals sleep? | journal = Trends in Neurosciences | volume = 31 | issue = 4 | pages = 208–13 | date = April 2008 | pmid = 18328577 | doi = 10.1016/j.tins.2008.02.001 | pmc = 8765194 | s2cid = 6614359 }}</ref>

Sleep increases the [[sensory threshold]]. In other words, sleeping persons perceive fewer stimuli, but can generally still respond to loud noises and other salient sensory events.<ref name="Siegel2008"/><ref name=MaquetEtAl2006 />

During [[slow-wave sleep]], humans secrete bursts of [[growth hormone]]. All sleep, even during the day, is associated with the secretion of [[prolactin]].<ref name=VanCauterSpiegel1999>{{cite book | vauthors = Van Cauter E, Spiegel K | date = 1999 | chapter = Circadian and Sleep Control of Hormonal Secretions | veditors = Zee PC, Turek FW | title = Regulation of Sleep and Circadian Rhythms | pages = 397–425 }}</ref>

Key physiological methods for monitoring and measuring changes during sleep include [[electroencephalography]] (EEG) of [[neural oscillation|brain waves]], [[electrooculography]] (EOG) of eye movements, and [[electromyography]] (EMG) of [[skeletal muscle]] activity. Simultaneous collection of these measurements is called [[polysomnography]], and can be performed in a specialized [[sleep medicine#Diagnostic methods|sleep laboratory]].<ref>[[#Brown|Brown]], p. 1087.</ref><ref name=Peraita-Adrados2005>{{cite book |doi=10.1142/9781860947186_0005 |chapter=Electroencephalography, Polysomnography, and Other Sleep Recording Systems |chapter-url=https://books.google.com/books?id=cxFqDQAAQBAJ&pg=PA103 |title=The Physiologic Nature of Sleep |year=2005 | vauthors = Peraita-Adrados R |pages=103–122 |publisher=Published by Imperial College Press and Distributed by World Scientific Publishing Co. |isbn=978-1-86094-557-1 }}</ref> Sleep researchers also use simplified [[electrocardiography]] (EKG) for cardiac activity and [[actigraphy]] for motor movements.<ref name=Peraita-Adrados2005 />

=== Brain waves in sleep ===
The electrical activity seen on an EEG represents brain waves. The amplitude of EEG waves at a particular frequency corresponds to various points in the sleep-wake cycle, such as being asleep, being awake, or falling asleep.<ref>{{citation|vauthors=Borbély AA, Daan S, Wirz-Justice A, Deboer T|title= The two-process model of sleep regulation: a reappraisal|journal= J Sleep Res | volume=25|issue=2|pages=131–43|doi= 10.1111/jsr.12371 |date=14 January 2016 |pmid=26762182|s2cid= 206156163|url= https://www.zora.uzh.ch/id/eprint/121344/1/TPM%20a%20reappraisal%2025-11-15%20Figs%20inserted%20in%20text-2.pdf}}</ref> Alpha, beta, theta, gamma, and delta waves are all seen in the different stages of sleep. Each waveform maintains a different frequency and amplitude. Alpha waves are seen when a person is in a resting state, but is still fully conscious. Their eyes may be closed and all of their body is resting and relatively still, where the body is starting to slow down. Beta waves take over alpha waves when a person is at attention, as they might be completing a task or concentrating on something. Beta waves consist of the highest of frequencies and the lowest of amplitude, and occur when a person is fully alert. Gamma waves are seen when a person is highly focused on a task or using all their concentration. Theta waves occur during the period of a person being awake, and they continue to transition into Stage 1 of sleep and in stage 2. Delta waves are seen in stages 3 and 4 of sleep when a person is in their deepest of sleep.<ref>{{citation|vauthors=Posada-Quintero HF, Reljin N, Bolkhovsky JB, Orjuela-Cañón AD, Chon KH|title=Brain Activity Correlates With Cognitive Performance Deterioration During Sleep Deprivation. |journal=Front. Neurosci. |date= 19 September 2019 |volume=13 |page=1001 |doi=10.3389/fnins.2019.01001|pmid=31607847 |pmc=6761229 |doi-access=free }}</ref>

===Non-REM and REM sleep===
Sleep is divided into two broad types: [[non-rapid eye movement sleep|non-rapid eye movement]] (non-REM or NREM) sleep and [[rapid eye movement sleep|rapid eye movement]] (REM) sleep. Non-REM and REM sleep are so different that physiologists identify them as distinct behavioral states. Non-REM sleep occurs first and after a transitional period is called [[slow-wave sleep]] or deep sleep. During this phase, body temperature and heart rate fall, and the brain uses less energy.<ref name=MaquetEtAl2006 /> REM sleep, also known as paradoxical sleep, represents a smaller portion of total sleep time. It is the main occasion for dreams (or [[nightmare]]s), and is associated with desynchronized and fast brain waves, eye movements, loss of muscle tone,<ref name="ninds">{{cite web |title=Brain Basics: Understanding Sleep |url=https://www.ninds.nih.gov/health-information/public-education/brain-basics/brain-basics-understanding-sleep |website=National Institute of Neurological Disorders and Stroke}}</ref> and suspension of [[homeostasis]].<ref>{{cite book | veditors = Parmeggiani PL | date = 2011 | title = Systemic Homeostasis and Poikilostasis in Sleep: Is REM Sleep a Physiological Paradox? | location = London | publisher = Imperial College Press | isbn = 978-1-94916-572-2 |pages=12–5}}</ref>

The [[sleep cycle]] of alternate NREM and REM sleep takes an average of 90 minutes, occurring 4–6 times in a good night's sleep.<ref name=Peraita-Adrados2005 /><ref name=McCarley2007>{{cite journal | vauthors = McCarley RW | title = Neurobiology of REM and NREM sleep | journal = Sleep Medicine | volume = 8 | issue = 4 | pages = 302–30 | date = June 2007 | pmid = 17468046 | doi = 10.1016/j.sleep.2007.03.005 | author-link = Robert W. McCarley }}</ref> The [[American Academy of Sleep Medicine]] (AASM) divides NREM into three stages: N1, N2, and N3, the last of which is also called delta sleep or [[slow-wave sleep]].<ref>{{cite journal | vauthors = Silber MH, Ancoli-Israel S, Bonnet MH, Chokroverty S, Grigg-Damberger MM, Hirshkowitz M, Kapen S, Keenan SA, Kryger MH, Penzel T, Pressman MR, Iber C | display-authors = 6 | title = The visual scoring of sleep in adults | journal = Journal of Clinical Sleep Medicine | volume = 3 | issue = 2 | pages = 121–31 | date = March 2007 | pmid = 17557422 | doi = 10.5664/jcsm.26814 | doi-access = free }}</ref> The whole period normally proceeds in the order: N1 → N2 → N3 → N2 → REM. REM sleep occurs as a person returns to stage 2 or 1 from a deep sleep.<ref name=ninds/> There is a greater amount of deep sleep (stage N3) earlier in the night, while the proportion of REM sleep increases in the two cycles just before natural awakening.<ref name=Peraita-Adrados2005 />

=== Awakening ===
{{Redirect|Waking up }}
{{Further|Wakefulness|Ascending reticular activating system}}
[[File:1900 The Awakening.jpg|thumb|"The Awakening", an illustration to writing by [[Leo&nbsp;Tolstoy]]
]]
Awakening can mean the end of sleep, or simply a moment to survey the environment and readjust body position before falling back asleep. Sleepers typically awaken soon after the end of a REM phase or sometimes in the middle of REM. Internal [[Circadian clock|circadian indicators]], along with a successful reduction of homeostatic sleep need, typically bring about awakening and the end of the sleep cycle.<ref name=AkerstedtEtAl2002>{{cite journal | vauthors = Akerstedt T, Billiard M, Bonnet M, Ficca G, Garma L, Mariotti M, Salzarulo P, Schulz H | display-authors = 6 | title = Awakening from sleep | journal = Sleep Medicine Reviews | volume = 6 | issue = 4 | pages = 267–86 | date = August 2002 | pmid = 12531132 | doi = 10.1053/smrv.2001.0202 | doi-access = free }}</ref> Awakening involves heightened electrical activation in the brain, beginning with the [[thalamus]] and spreading throughout the [[Cerebral cortex|cortex]].<ref name=AkerstedtEtAl2002 />

On a typical night of sleep, there is not much time that is spent in the waking state. In various sleep studies that have been conducted using the electroencephalography, it has been found that females are awake for 0–1% during their nightly sleep while males are awake for 0–2% during that time. In adults, wakefulness increases, especially in later cycles. One study found 3% awake time in the first ninety-minute sleep cycle, 8% in the second, 10% in the third, 12% in the fourth, and 13–14% in the fifth. Most of this awake time occurred shortly after REM sleep.<ref name=AkerstedtEtAl2002 />

Today, many humans wake up with an [[alarm clock]];<ref name=RonnenbergEtAl2007>{{cite journal | vauthors = Roenneberg T, Kuehnle T, Juda M, Kantermann T, Allebrandt K, Gordijn M, Merrow M | title = Epidemiology of the human circadian clock | journal = Sleep Medicine Reviews | volume = 11 | issue = 6 | pages = 429–38 | date = December 2007 | pmid = 17936039 | doi = 10.1016/j.smrv.2007.07.005 | url = https://pure.rug.nl/ws/files/6712602/2007SleepMedRevRoenneberg.pdf | hdl = 11370/65d6f03a-88cd-405c-a067-4afbc1b9ba9d | s2cid = 11628329 | hdl-access = free }}</ref> however, people can also reliably wake themselves up at a specific time with no need for an alarm.<ref name=AkerstedtEtAl2002 /> Many sleep quite differently on workdays versus days off, a pattern which can lead to chronic circadian desynchronization.<ref name=WaterhouseEtAl2012>{{cite journal | vauthors = Waterhouse J, Fukuda Y, Morita T | title = Daily rhythms of the sleep-wake cycle | journal = Journal of Physiological Anthropology | volume = 31 | issue = 5 | article-number = 5 | date = March 2012 | pmid = 22738268 | pmc = 3375033 | doi = 10.1186/1880-6805-31-5 | doi-access = free }}</ref><ref name=RonnenbergEtAl2007 /> Many people regularly look at television and other screens before going to bed, a factor which may exacerbate disruption of the circadian cycle.<ref name="ChangAM" /><ref>{{cite journal | vauthors = Basner M, Dinges DF | title = Dubious bargain: trading sleep for Leno and Letterman | journal = Sleep | volume = 32 | issue = 6 | pages = 747–52 | date = June 2009 | pmid = 19544750 | pmc = 2690561 | doi = 10.1093/sleep/32.6.747 }}</ref> Scientific studies on sleep have shown that sleep stage at awakening is an important factor in amplifying [[sleep inertia]].<ref>{{cite journal | vauthors = Tassi P, Muzet A | title = Sleep inertia | journal = Sleep Medicine Reviews | volume = 4 | issue = 4 | pages = 341–353 | date = August 2000 | pmid = 12531174 | doi = 10.1053/smrv.2000.0098 }}</ref>

Determinants of [[alertness]] after waking up include quantity/quality of the sleep, physical activity the day prior, a carbohydrate-rich breakfast, and a low [[Blood sugar level|blood glucose]] response to it.<ref>{{cite journal |last1=Vallat |first1=Raphael |last2=Berry |first2=Sarah E. |last3=Tsereteli |first3=Neli |last4=Capdevila |first4=Joan |last5=Khatib |first5=Haya Al |last6=Valdes |first6=Ana M. |last7=Delahanty |first7=Linda M. |last8=Drew |first8=David A. |last9=Chan |first9=Andrew T. |last10=Wolf |first10=Jonathan |last11=Franks |first11=Paul W. |last12=Spector |first12=Tim D. |last13=Walker |first13=Matthew P. |title=How people wake up is associated with previous night's sleep together with physical activity and food intake |journal=Nature Communications |date=19 November 2022 |volume=13 |issue=1 |page=7116 |doi=10.1038/s41467-022-34503-2 |pmid=36402781 |pmc=9675783 |bibcode=2022NatCo..13.7116V |language=en |issn=2041-1723|doi-access=free}}</ref>

== Timing ==
Sleep timing is controlled by the [[circadian clock]] (Process C), sleep-wake [[homeostasis]] (Process S), and to some extent by the individual will.

=== Circadian clock ===
{{Main|Circadian rhythm}}
{{further|Circadian rhythm sleep disorder}}
[[image:Biological clock human.svg|402px|thumb|The human "[[Circadian rhythm|biological clock]]"]]

Sleep timing depends greatly on [[hormone|hormonal]] signals from the circadian clock, or Process C, a complex neurochemical system which uses signals from an organism's environment to recreate an internal day–night rhythm. Process C counteracts the homeostatic drive for sleep during the day (in [[diurnality|diurnal]] animals) and augments it at night.<ref name=FullerEtAl2006>{{cite journal | vauthors = Fuller PM, Gooley JJ, Saper CB | title = Neurobiology of the sleep-wake cycle: sleep architecture, circadian regulation, and regulatory feedback | journal = Journal of Biological Rhythms | volume = 21 | issue = 6 | pages = 482–93 | date = December 2006 | pmid = 17107938 | doi = 10.1177/0748730406294627 | s2cid = 36572447 }}</ref><ref name=WaterhouseEtAl2012 /> The [[suprachiasmatic nucleus]] (SCN), a brain area directly above the [[optic chiasm]], is presently considered the most important nexus for this process; however, secondary clock systems have been found throughout the body.

An organism whose circadian clock exhibits a regular rhythm corresponding to outside signals is said to be ''[[Entrainment (chronobiology)|entrained]]''; an entrained rhythm persists even if the outside signals suddenly disappear. If an entrained human is isolated in a bunker with constant light or darkness, he or she will continue to experience rhythmic increases and decreases of body temperature and melatonin, on a period that slightly exceeds 24 hours. Scientists refer to such conditions as [[Free-running sleep|free-running]] of the circadian rhythm. Under natural conditions, light signals regularly adjust this period downward, so that it corresponds better with the exact 24 hours of an Earth day.<ref name=RonnenbergEtAl2007 /><ref>{{cite book | author-link1 = Phyllis C. Zee | vauthors = Zee PC, Turek FW | date = 1999 | chapter = Introduction to Sleep and Circadian Rhythms | veditors = Zee PC, Turek FW | title = Regulation of Sleep and Circadian Rhythms | pages = 1–17 }}</ref><ref name=DijkEdgar1999>{{cite book | vauthors = Derk-Jan D, Edgar DM | date = 1999 | chapter = Circadian and Homeostatic Control of Wakefulness and Sleep | veditors = Zee PC, Turek FW | title = Regulation of Sleep and Circadian Rhythms | pages = 111–147 }}</ref>

The circadian clock exerts constant influence on the body, affecting [[Sine wave|sinusoidal]] oscillation of [[thermoregulation|body temperature]] between roughly 36.2&nbsp;°C and 37.2&nbsp;°C.<ref name=DijkEdgar1999 /><ref name=CzeislerWright1999>{{cite book | vauthors = Czeisler CA, Wright Jr KP | date = 1999 | chapter = Influence of Light on Circadian Rhythmicity in Humans | veditors = Zee PC, Turek FW | title = Regulation of Sleep and Circadian Rhythms | pages = 149–180 }}</ref> The suprachiasmatic nucleus itself shows conspicuous oscillation activity, which intensifies during subjective day (i.e., the part of the rhythm corresponding with daytime, whether accurately or not) and drops to almost nothing during subjective night.<ref name=ZlomanczukSchwartz1999>{{cite book | vauthors = Zlomanczuk P, Schwartz WJ | date = 1999 | chapter = Cellular and Molecular Mechanisms of Circadian Rhythms in Mammals | veditors = Zee PC, Turek FW | title = Regulation of Sleep and Circadian Rhythms | pages = 309–342 }}</ref> The circadian pacemaker in the suprachiasmatic nucleus has a direct neural connection to the [[pineal gland]], which releases the hormone [[melatonin]] at night.<ref name=ZlomanczukSchwartz1999 /> [[Cortisol]] levels typically rise throughout the night, [[Cortisol awakening response|peak in the awakening hours]], and diminish during the day.<ref name="VanCauterSpiegel1999"/><ref name=Wehr1999>{{cite book | vauthors = Wehr TA | date = 1999 | chapter = The Impact of Changes in Nightlength (Scotoperiod) on Human Sleep | veditors = Zee PC, Turek FW | title = Regulation of Sleep and Circadian Rhythms | pages = 263–285 }}</ref> Circadian [[prolactin]] secretion begins in the late afternoon, especially in women, and is subsequently augmented by sleep-induced secretion, to peak in the middle of the night. Circadian rhythm exerts some influence on the nighttime secretion of growth hormone.<ref name=VanCauterSpiegel1999 />

The circadian rhythm influences the ideal timing of a restorative sleep episode.<ref name=RonnenbergEtAl2007 /><ref name=Wyatt1999>{{cite journal | vauthors = Wyatt JK, Ritz-De Cecco A, Czeisler CA, Dijk DJ | title = Circadian temperature and melatonin rhythms, sleep, and neurobehavioral function in humans living on a 20-h day | journal = The American Journal of Physiology | volume = 277 | issue = 4 Pt 2 | pages = R1152-63 | date = October 1999 | pmid = 10516257 | doi = 10.1152/ajpregu.1999.277.4.r1152 | s2cid = 4474347 | quote = ... significant homeostatic and circadian modulation of sleep structure, with the highest sleep efficiency occurring in sleep episodes bracketing the melatonin maximum and core body temperature minimum <!-- the quote is only relevant to one time that this ref is used --> }}</ref> Sleepiness increases during the night. REM sleep occurs more during body temperature minimum within the circadian cycle, whereas [[slow-wave sleep]] can occur more independently of circadian time.<ref name=DijkEdgar1999 />

The internal circadian clock is profoundly influenced by changes in light, since these are its main clues about what time it is. Exposure to even small amounts of light during the night can suppress melatonin secretion, and increase body temperature and wakefulness. Short pulses of light, at [[phase response curve|the right moment]] in the circadian cycle, can significantly 'reset' the internal clock.<ref name=CzeislerWright1999 /> Blue light, in particular, exerts the strongest effect,<ref name=WaterhouseEtAl2012 /> leading to concerns that [[Electronic media and sleep|use of a screen]] before bed may interfere with sleep.<ref name="ChangAM">{{cite journal | vauthors = Chang AM, Aeschbach D, Duffy JF, Czeisler CA | title = Evening use of light-emitting eReaders negatively affects sleep, circadian timing, and next-morning alertness | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 112 | issue = 4 | pages = 1232–7 | date = January 2015 | pmid = 25535358 | pmc = 4313820 | doi = 10.1073/pnas.1418490112 | bibcode = 2015PNAS..112.1232C | doi-access = free }}</ref>

Modern humans often find themselves desynchronized from their internal circadian clock, due to the requirements of work (especially [[Shift work|night shifts]]), long-distance travel, and the influence of universal indoor lighting.<ref name=DijkEdgar1999 /> Even if they have sleep debt, or feel sleepy, people can have difficulty staying asleep at the peak of their circadian cycle. Conversely, they can have difficulty waking up in the trough of the cycle.<ref name=AkerstedtEtAl2002 /> A healthy young adult entrained to the sun will (during most of the year) fall asleep a few hours after sunset, experience body temperature minimum at 6 a.m., and wake up a few hours after sunrise.<ref name=DijkEdgar1999 />

=== Process S ===
{{Main|Sleep debt}}
Generally speaking, the longer an organism is awake, the more it feels a need to sleep ("sleep debt"). This driver of sleep is referred to as '''Process S'''. The balance between sleeping and waking is regulated by a process called [[homeostasis]]. Induced or perceived lack of sleep is called [[sleep deprivation]].

Process S is driven by the depletion of [[glycogen]] and accumulation of [[adenosine]] in the forebrain that disinhibits the [[ventrolateral preoptic nucleus]], allowing for inhibition of the [[ascending reticular activating system]].<ref>{{cite journal | vauthors = Schwartz JR, Roth T | title = Neurophysiology of sleep and wakefulness: basic science and clinical implications | journal = Current Neuropharmacology | volume = 6 | issue = 4 | pages = 367–78 | date = December 2008 | pmid = 19587857 | pmc = 2701283 | doi = 10.2174/157015908787386050 }}</ref>

Sleep deprivation tends to cause slower brain waves in the [[frontal cortex]], shortened attention span, higher anxiety, impaired memory, and a grouchy [[Mood (psychology)|mood]]. Conversely, a well-rested organism tends to have improved memory and mood.<ref>[[#Brown|Brown]], pp. 1134–1138.</ref> Neurophysiological and functional [[imaging studies]] have demonstrated that frontal regions of the brain are particularly responsive to homeostatic sleep pressure.<ref>{{cite journal | vauthors = Gottselig JM, Adam M, Rétey JV, Khatami R, Achermann P, Landolt HP | title = Random number generation during sleep deprivation: effects of caffeine on response maintenance and stereotypy | journal = Journal of Sleep Research | volume = 15 | issue = 1 | pages = 31–40 | date = March 2006 | pmid = 16490000 | doi = 10.1111/j.1365-2869.2006.00497.x | s2cid = 10355305 }}</ref>

There is disagreement on how much sleep debt is possible to accumulate, and whether sleep debt is accumulated against an individual's average sleep or some other benchmark. It is also unclear whether the prevalence of sleep debt among adults has changed appreciably in the [[developed country|industrialized world]] in recent decades. Sleep debt does show some evidence of being cumulative. Subjectively, however, humans seem to reach maximum sleepiness 30 hours after waking.<ref name=DijkEdgar1999 /> It is likely that in [[Western world|Western societies]], children are sleeping less than they previously have.<ref>{{cite journal | vauthors = Iglowstein I, Jenni OG, Molinari L, Largo RH | title = Sleep duration from infancy to adolescence: reference values and generational trends | journal = Pediatrics | volume = 111 | issue = 2 | pages = 302–7 | date = February 2003 | pmid = 12563055 | doi = 10.1542/peds.111.2.302 | quote = Thus, the shift in the evening bedtime across cohorts accounted for the substantial decrease in sleep duration in younger children between the 1970s and the 1990s... [A] more liberal parental attitude toward evening bedtime in the past decades is most likely responsible for the bedtime shift and for the decline of sleep duration... | s2cid = 8727836 }}</ref>

One neurochemical indicator of sleep debt is [[adenosine]], a neurotransmitter that inhibits many of the bodily processes associated with wakefulness. Adenosine levels increase in the cortex and basal forebrain during prolonged wakefulness, and decrease during the sleep-recovery period, potentially acting as a homeostatic regulator of sleep.<ref name="pmid25175972">{{cite journal | vauthors = Huang ZL, Zhang Z, Qu WM | title = Roles of adenosine and its receptors in sleep-wake regulation | journal = International Review of Neurobiology | volume = 119 | pages = 349–71 | date = 2014 | pmid = 25175972 | doi = 10.1016/B978-0-12-801022-8.00014-3 | isbn = 978-0-12-801022-8 }}</ref><ref>{{cite web|url=https://thebrain.mcgill.ca/flash/a/a_11/a_11_m/a_11_m_cyc/a_11_m_cyc.html|title=The brain from top to bottom: Molecules that build up and make you sleep|publisher=McGill University, Montreal, Quebec, Canada|access-date=20 September 2012|archive-url=https://web.archive.org/web/20130207001251/https://thebrain.mcgill.ca/flash/a/a_11/a_11_m/a_11_m_cyc/a_11_m_cyc.html|archive-date=7 February 2013|url-status=live}}</ref> [[Coffee]], tea, and other sources of [[caffeine]] temporarily block the effect of adenosine, prolong sleep latency, and reduce total sleep time and quality.<ref>{{cite journal | vauthors = Clark I, Landolt HP | title = Coffee, caffeine, and sleep: A systematic review of epidemiological studies and randomized controlled trials | journal = Sleep Medicine Reviews | volume = 31 | pages = 70–78 | date = February 2017 | pmid = 26899133 | doi = 10.1016/j.smrv.2016.01.006 | url = https://www.zora.uzh.ch/id/eprint/127096/14/Clark_Landolt_Sleep%20Med%20Rev%20%282016%29.pdf | access-date = 19 November 2018 | url-status = live | archive-url = https://web.archive.org/web/20181104121236/https://www.zora.uzh.ch/id/eprint/127096/14/Clark_Landolt_Sleep%20Med%20Rev%20(2016).pdf | archive-date = 4 November 2018 | doi-access = free }}</ref>

=== Social timing ===
Humans are also influenced by aspects of ''social time'', such as the hours when other people are awake, the hours when work is required, the time on clocks, etc. [[Time zone]]s, standard times used to unify the timing for people in the same area, correspond only approximately to the natural rising and setting of the sun. An extreme example of the approximate nature of time zones is China, a country which used to span five time zones and now [[Time in China|officially uses only one]] (UTC+8).<ref name=RonnenbergEtAl2007 />

=== Distribution ===
In [[polyphasic sleep]], an organism sleeps several times in a 24-hour cycle, whereas in monophasic sleep this occurs all at once. Under experimental conditions, humans tend to alternate more frequently between sleep and wakefulness (i.e., exhibit more polyphasic sleep) if they have nothing better to do.<ref name=DijkEdgar1999 /> Given a 14-hour period of darkness in experimental conditions, humans tended towards bimodal sleep, with two sleep periods concentrated at the beginning and at the end of the dark time. Bimodal sleep in humans was more common before the [[Industrial Revolution]].<ref name=Wehr1999 />

Different characteristic sleep patterns, such as the familiarly so-called "[[Lark (person)|early bird]]" and "[[Night owl (person)|night owl]]", are called ''[[chronotype]]s''. Genetics and sex have some influence on chronotype, but so do habits. Chronotype is also liable to change over the course of a person's lifetime. Seven-year-olds are better disposed to wake up early in the morning than are fifteen-year-olds.<ref name=WaterhouseEtAl2012 /><ref name=RonnenbergEtAl2007 /> Chronotypes far outside the normal range are called circadian rhythm sleep disorders.<ref>{{cite journal | vauthors = Dagan Y | title = Circadian rhythm sleep disorders (CRSD) | journal = Sleep Medicine Reviews | volume = 6 | issue = 1 | pages = 45–54 | date = February 2002 | pmid = 12531141 | doi = 10.1053/smrv.2001.0190 | url = https://www.neurosono.com.br/arquivos/1155473343.pdf | access-date = 5 June 2016 | quote = Early onset of CRSD, the ease of diagnosis, the high frequency of misdiagnosis and erroneous treatment, the potentially harmful psychological and adjustment consequences, and the availability of promising treatments, all indicate the importance of greater awareness of these disorders. | format = PDF: full text | archive-url = https://web.archive.org/web/20080227161654/https://www.neurosono.com.br/arquivos/1155473343.pdf | archive-date = 27 February 2008 }}</ref>

====Naps====
{{main|Nap}}Naps are short periods of sleep that one might take during the daytime, often in order to get the necessary amount of rest. Napping is often associated with childhood, but around one-third of American adults partake in it daily. The optimal nap duration is around 10–20 minutes, as researchers have proven that it takes at least 30 minutes to enter slow-wave sleep, the deepest period of sleep.<ref name=":0">Fry, A. (9 October 2020). ''Napping: Health Benefits & Tips for your best nap''. Sleep Foundation. Retrieved 14 November 2021, from https://www.sleepfoundation.org/sleep-hygiene/napping .</ref> Napping too long and entering the slow wave cycles can make it difficult to awake from the nap and leave one feeling unrested. This period of drowsiness is called [[sleep inertia]].

[[File:Hombre echando una siesta en San Cristóbal, Cusco, Perú, 2015-07-31, DD 49.JPG|thumb|Man napping in San Cristobal, [[Peru]]]]
The [[siesta]] habit has recently been associated with a 37% lower coronary mortality, possibly due to reduced cardiovascular stress mediated by daytime sleep.<ref name=Naska>{{cite journal | vauthors = Naska A, Oikonomou E, Trichopoulou A, Psaltopoulou T, Trichopoulos D | title = Siesta in healthy adults and coronary mortality in the general population | journal = Archives of Internal Medicine | volume = 167 | issue = 3 | pages = 296–301 | date = February 2007 | pmid = 17296887 | doi = 10.1001/archinte.167.3.296 | doi-access = free }}</ref> Short naps at mid-day and mild evening exercise were found to be effective for improved sleep, cognitive tasks, and mental health in elderly people.<ref>{{cite journal | vauthors = Tanaka H, Tamura N | title = Sleep education with self-help treatment and sleep health promotion for mental and physical wellness in Japan | journal = Sleep and Biological Rhythms | volume = 14 | issue = S1 | pages = 89–99 | date = January 2016 | pmid = 26855610 | pmc = 4732678 | doi = 10.1007/s41105-015-0018-6 }}</ref>

=== Genetics ===
Monozygotic (identical) but not dizygotic (fraternal) twins tend to have similar sleep habits. Neurotransmitters, molecules whose production can be traced to specific genes, are one genetic influence on sleep that can be analyzed. The circadian clock has its own set of genes.<ref>[[#Brown|Brown]], pp. 1138–1102.</ref> Genes which may influence sleep include [[ABCC9]], [[DEC2]], [[Dopamine receptor D2]]<ref name="The Molecular Genetics of Human Sleep">{{cite journal | vauthors = Zhang L, Fu YH | title = The molecular genetics of human sleep | journal = The European Journal of Neuroscience | volume = 51 | issue = 1 | pages = 422–428 | date = January 2020 | pmid = 30144347 | pmc = 6389443 | doi = 10.1111/ejn.14132 }}</ref> and variants near [[PAX8|PAX 8]] and [[VRK2]].<ref name=pmid27494321>{{cite journal | vauthors = Jones SE, Tyrrell J, Wood AR, Beaumont RN, Ruth KS, Tuke MA, Yaghootkar H, Hu Y, Teder-Laving M, Hayward C, Roenneberg T, Wilson JF, Del Greco F, Hicks AA, Shin C, Yun CH, Lee SK, Metspalu A, Byrne EM, Gehrman PR, Tiemeier H, Allebrandt KV, Freathy RM, Murray A, Hinds DA, Frayling TM, Weedon MN | display-authors = 6 | title = Genome-Wide Association Analyses in 128,266 Individuals Identifies New Morningness and Sleep Duration Loci | journal = PLOS Genetics | volume = 12 | issue = 8 | article-number = e1006125 | date = August 2016 | pmid = 27494321 | pmc = 4975467 | doi = 10.1371/journal.pgen.1006125 | doi-access = free }}</ref> While the latter have been found in a [[Genome-wide association study|GWAS study]] that primarily detects correlations (but not necessarily causation), other genes have been shown to have a more direct effect. For instance, mice lacking [[Dihydropyrimidine dehydrogenase (NADP+)|dihydropyrimidine dehydrogenase]] (Dpyd) had 78.4 min less sleep during the lights-off period than wild-type mice. Dpyd encodes the rate-limiting [[enzyme]] in the metabolic pathway that catabolizes [[uracil]] and [[thymidine]] to β-[[alanine]], an inhibitory [[neurotransmitter]]. This also supports the role of β-alanine as a neurotransmitter that promotes sleep in mice.<ref>{{cite journal | vauthors = Keenan BT, Galante RJ, Lian J, Zhang L, Guo X, Veatch OJ, Chesler EJ, O'Brien WT, Svenson KL, Churchill GA, Pack AI | display-authors = 6 | title = The dihydropyrimidine dehydrogenase gene contributes to heritable differences in sleep in mice | journal = Current Biology | volume = 31 | issue = 23 | pages = 5238–5248.e7 | date = December 2021 | pmid = 34653361 | pmc = 8665053 | doi = 10.1016/j.cub.2021.09.049 | bibcode = 2021CBio...31E5238K | s2cid = 238754563 }}</ref>

==== Genes for short sleep duration ====
{{Excerpt|Familial natural short sleep}} The genes [[DEC2]], [[ADRB1]], [[NPSR1]] and [[GRM1]] are implicated in enabling short sleep.<ref>{{cite journal |last1=Zheng |first1=Liubin |last2=Zhang |first2=Luoying |title=The molecular mechanism of natural short sleep: A path towards understanding why we need to sleep |journal=Brain Science Advances |date=September 2022 |volume=8 |issue=3 |pages=165–172 |doi=10.26599/BSA.2022.9050003 |s2cid=250363367 |language=en |issn=2096-5958|doi-access=free }}</ref>

=== Quality ===
The quality of sleep may be evaluated from an objective and a subjective point of view. Objective sleep quality refers to how difficult it is for a person to fall asleep and remain in a sleeping state, and how many times they wake up during a single night. Poor sleep quality disrupts the cycle of transition between the different stages of sleep.<ref>{{cite journal | vauthors = Barnes CM, Lucianetti L, Bhave DP, Christian MS |year=2015 |title=You wouldn't like me when I'm sleepy: Leaders' sleep, daily abusive supervision, and work unit engagement |journal=Academy of Management Journal |volume=58 |issue=5 |pages=1419–1437 |doi=10.5465/amj.2013.1063 |s2cid=145056840 |url=https://ink.library.smu.edu.sg/lkcsb_research/4353 }}</ref> Subjective sleep quality in turn refers to a sense of being rested and regenerated after awaking from sleep. A study by A. Harvey et al. (2002) found that insomniacs were more demanding in their evaluations of sleep quality than individuals who had no sleep problems.<ref>{{cite journal | vauthors = Harvey AG, Payne S | title = The management of unwanted pre-sleep thoughts in insomnia: distraction with imagery versus general distraction | journal = Behaviour Research and Therapy | volume = 40 | issue = 3 | pages = 267–77 | date = March 2002 | pmid = 11863237 | doi = 10.1016/s0005-7967(01)00012-2 | s2cid = 16647017 }}</ref>

Homeostatic sleep propensity (the need for sleep as a function of time elapsed since the last adequate sleep episode) must be balanced against the circadian element for satisfactory sleep.<ref name="Zisapel">{{cite journal | vauthors = Zisapel N | title = Sleep and sleep disturbances: biological basis and clinical implications | journal = Cellular and Molecular Life Sciences | volume = 64 | issue = 10 | pages = 1174–86 | date = May 2007 | pmid = 17364142 | doi = 10.1007/s00018-007-6529-9 | s2cid = 2003308 | pmc = 11136060 }}</ref><ref>{{cite journal | vauthors = Dijk DJ, Lockley SW | title = Integration of human sleep-wake regulation and circadian rhythmicity | journal = Journal of Applied Physiology | volume = 92 | issue = 2 | pages = 852–62 | date = February 2002 | pmid = 11796701 | doi = 10.1152/japplphysiol.00924.2001 | quote = Consolidation of sleep for 8 h or more is only observed when sleep is initiated ~6–8 h before the temperature nadir. | s2cid = 2502686 }}</ref> Along with corresponding messages from the circadian clock, this tells the body it needs to sleep.<ref name="autogenerated1">{{cite web|url=https://www.helpguide.org/life/sleeping.htm |title=Understanding Sleep: Sleep Needs, Cycles, and Stages |access-date=25 January 2008 | vauthors = de Benedictis T, Larson H, Kemp G, Barston S, Segal R |year=2007 |publisher=Helpguide.org |archive-url=https://web.archive.org/web/20080124000744/https://www.helpguide.org/life/sleeping.htm |archive-date=24 January 2008 }}</ref> The timing is correct when the following two circadian markers occur after the middle of the sleep episode and before awakening:<ref name=Wyatt1999 /> maximum concentration of the hormone melatonin, and minimum core body temperature.

== Ideal duration ==
[[File:2023 CDC recommendations for amount of sleep needed, by age.svg |thumb|Centers for Disease Control and Prevention (CDC) recommendations for sleep needed decrease with age.<ref name=CDC_sleep>{{cite web |title=How Much Sleep Do I Need? |url=https://www.cdc.gov/sleep/about_sleep/how_much_sleep.html |website=CDC.gov |date=14 September 2022 |publisher=Centers for Disease Control and Prevention (CDC) |archive-url=https://web.archive.org/web/20231102143914/https://www.cdc.gov/sleep/about_sleep/how_much_sleep.html |archive-date=2 November 2023 |quote=Last Reviewed: September 14, 2022. Source: National Center for Chronic Disease Prevention and Health Promotion, Division of Population Health. |url-status=live }}</ref>]]
[[File:Effects of sleep deprivation.svg |thumb |The main health effects of [[sleep deprivation]],<ref>Reference list is found on image page in Commons: [[:Commons:File:Effects of sleep deprivation.svg#References]]</ref> indicating impairment of normal maintenance by sleep]]

Human sleep-needs vary by age and amongst individuals;<ref>{{Cite journal |last1=Hirshkowitz |first1=Max |last2=Whiton |first2=Kaitlyn |last3=Albert |first3=Steven M. |last4=Alessi |first4=Cathy |last5=Bruni |first5=Oliviero |last6=DonCarlos |first6=Lydia |last7=Hazen |first7=Nancy |last8=Herman |first8=John |last9=Katz |first9=Eliot S. |last10=Kheirandish-Gozal |first10=Leila |last11=Neubauer |first11=David N. |last12=O'Donnell |first12=Anne E. |last13=Ohayon |first13=Maurice |last14=Peever |first14=John |last15=Rawding |first15=Robert |date=March 2015 |title=National Sleep Foundation's sleep time duration recommendations: methodology and results summary |journal=Sleep Health |volume=1 |issue=1 |pages=40–43 |doi=10.1016/j.sleh.2014.12.010 |issn=2352-7226 |pmid=29073412|s2cid=205190733 }}</ref> sleep is considered to be adequate when there is no daytime sleepiness or dysfunction.<ref>{{Cite journal |last=Benbadis |first=S. R. |date=November 1998 |title=Daytime sleepiness: when is it normal? When to refer? |journal=Cleveland Clinic Journal of Medicine |volume=65 |issue=10 |pages=543–549 |doi=10.3949/ccjm.65.10.543 |doi-broken-date=1 July 2025 |issn=0891-1150 |pmid=9830788|s2cid=8222974 }}</ref> Moreover, self-reported sleep duration is only moderately correlated with actual sleep time as measured by [[actigraphy]],<ref>{{cite journal | vauthors = Lauderdale DS, Knutson KL, Yan LL, Liu K, Rathouz PJ | title = Self-reported and measured sleep duration: how similar are they? | journal = Epidemiology | volume = 19 | issue = 6 | pages = 838–45 | date = November 2008 | pmid = 18854708 | pmc = 2785092 | doi = 10.1097/EDE.0b013e318187a7b0 }}</ref> and those affected with [[sleep state misperception]] may typically report having slept only four hours despite having slept a full eight hours.<ref name=healthcom>[https://www.sleepdisorderchannel.com/insomnia/causes.shtml Insomnia Causes] {{Webarchive|url=https://web.archive.org/web/20101022083445/https://www.sleepdisorderchannel.com/insomnia/causes.shtml |date=22 October 2010 }}. Healthcommunities.com. Original Publication: 1 December 2000, Updated: 1 December 2007.</ref><ref>{{Cite journal |last1=Arditte Hall |first1=Kimberly A. |last2=Werner |first2=Kimberly B. |last3=Griffin |first3=Michael G. |last4=Galovski |first4=Tara E. |date=10 January 2022 |title=Exploring Predictors of Sleep State Misperception in Women with Posttraumatic Stress Disorder |journal=Behavioral Sleep Medicine |volume=21 |issue=1 |pages=22–32 |doi=10.1080/15402002.2021.2024193 |issn=1540-2010 |pmc=9271136 |pmid=35007171 }}</ref><ref>{{Cite journal |last1=Truzzi |first1=Giselle de Martin |last2=Teixeira |first2=Igor de Lima |last3=do Prado |first3=Lucila Bizari Fernandes |last4=do Prado |first4=Gilmar Fernandes |last5=Tufik |first5=Sergio |last6=Coelho |first6=Fernando Morgadinho |date=January 2021 |title=Sleep state misperception: is there a CNS structural source? |journal=Sleep Science (Sao Paulo, Brazil) |volume=14 |issue=Spec 1 |pages=94–96 |doi=10.5935/1984-0063.20200039 |issn=1984-0659 |pmc=8663728 |pmid=34917280}}</ref>

Researchers have found that sleeping 6–7 hours each night correlates with longevity and cardiac health in humans, though many underlying factors may be involved in the causality behind this relationship.<ref>{{cite news | vauthors = Rowland R |title=Experts challenge study linking sleep, life span |date=15 February 2002 |url=https://edition.cnn.com/2002/HEALTH/02/14/sleep.study/index.html |access-date=29 October 2013 |work=CNN |archive-url=https://web.archive.org/web/20121005172421/https://edition.cnn.com/2002/HEALTH/02/14/sleep.study/index.html |archive-date=5 October 2012 |url-status=live }}</ref><ref>{{cite journal | vauthors = Patel SR, Ayas NT, Malhotra MR, White DP, Schernhammer ES, Speizer FE, Stampfer MJ, Hu FB | display-authors = 6 | title = A prospective study of sleep duration and mortality risk in women | journal = Sleep | volume = 27 | issue = 3 | pages = 440–4 | date = May 2004 | pmid = 15164896 | doi = 10.1093/sleep/27.3.440 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Patel SR, Malhotra A, Gottlieb DJ, White DP, Hu FB | title = Correlates of long sleep duration | journal = Sleep | volume = 29 | issue = 7 | pages = 881–9 | date = July 2006 | pmid = 16895254 | pmc = 3500381 | doi = 10.1093/sleep/29.7.881 }}; cf. {{cite journal | vauthors = Irwin MR, Ziegler M | title = Sleep deprivation potentiates activation of cardiovascular and catecholamine responses in abstinent alcoholics | journal = Hypertension | volume = 45 | issue = 2 | pages = 252–7 | date = February 2005 | pmid = 15642774 | doi = 10.1161/01.HYP.0000153517.44295.07 | s2cid = 2205895 | citeseerx = 10.1.1.535.7089 }}</ref><ref>{{cite journal|vauthors=Ferrie JE, Shipley MJ, Cappuccio FP, Brunner E, Miller MA, Kumari M, Marmot MG|date=December 2007|title=A prospective study of change in sleep duration: associations with mortality in the Whitehall II cohort|journal=Sleep|volume=30|issue=12|pages=1659–66|doi=10.1093/sleep/30.12.1659|pmc=2276139|pmid=18246975}}</ref><ref name=pmid27494321/><ref>{{cite journal | vauthors = Cappuccio FP, Taggart FM, Kandala NB, Currie A, Peile E, Stranges S, Miller MA | title = Meta-analysis of short sleep duration and obesity in children and adults | journal = Sleep | volume = 31 | issue = 5 | pages = 619–26 | date = May 2008 | pmid = 18517032 | pmc = 2398753 | doi = 10.1093/sleep/31.5.619 }}</ref><ref>{{cite journal | vauthors = Schmid SM, Hallschmid M, Schultes B | title = The metabolic burden of sleep loss | journal = The Lancet. Diabetes & Endocrinology | volume = 3 | issue = 1 | pages = 52–62 | date = January 2015 | pmid = 24731536 | doi = 10.1016/S2213-8587(14)70012-9 }}</ref>

Sleep difficulties are furthermore associated with psychiatric disorders such as [[major depressive disorder|depression]], [[alcoholism]], and [[bipolar disorder]].<ref name="Thase2006">{{cite journal | vauthors = Thase ME | title = Depression and sleep: pathophysiology and treatment | journal = Dialogues in Clinical Neuroscience | volume = 8 | issue = 2 | pages = 217–26 | date = 2006 | pmid = 16889107 | pmc = 3181772 | doi = 10.31887/DCNS.2006.8.2/mthase }}</ref> Up to 90 percent of adults with depression are found to have sleep difficulties. Dysregulation detected by EEG includes disturbances in sleep continuity, decreased delta sleep and altered REM patterns with regard to latency, distribution across the night and density of eye movements.<ref>{{cite book | vauthors = Mann JJ, Kupfer DJ |title=Biology of Depressive Disorders: Subtypes of depression and comorbid disorders, Part 2 |url=https://books.google.com/books?id=qbbTmje6oskC |format=Google books |access-date=24 July 2009 |year=1993 |publisher=Springer |page=49 |isbn=978-0-306-44296-4 |archive-url=https://web.archive.org/web/20170310194247/https://books.google.com/books?id=qbbTmje6oskC&printsec=frontcover |archive-date=10 March 2017 |url-status=live }}</ref>

Sleep duration can also vary according to season. Up to 90% of people report longer sleep duration in winter, which may lead to more pronounced [[seasonal affective disorder]].<ref>{{cite journal | vauthors = Suzuki M, Taniguchi T, Furihata R, Yoshita K, Arai Y, Yoshiike N, Uchiyama M | title = Seasonal changes in sleep duration and sleep problems: A prospective study in Japanese community residents | journal = PLOS ONE| volume = 14 | issue = 4 | article-number = e0215345 | date = 18 April 2019 | pmid = 30998709 | pmc = 6472875 | doi = 10.1371/journal.pone.0215345 | bibcode = 2019PLoSO..1415345S | doi-access = free }}</ref><ref>{{cite web |date=30 January 2023 |title=Hate waking up when it's dark out? Find out how winter really affects your sleep habits. |url=https://www.sleep.org/do-sleep-needs-change-with-seasons/ |archive-url=https://web.archive.org/web/20210302123736/https://www.sleep.org/do-sleep-needs-change-with-seasons/ |archive-date=2 March 2021 |work=Sleep.org}}</ref>

=== Children ===
{{See also|Infant sleep|Adolescent sleep}}
[[File:WLA metmuseum Bronze statue of Eros sleeping 7.jpg|thumb|Bronze statue of [[Eros]] sleeping, 3rd century BC–early 1st century AD]]

By the time infants reach the age of two, their brain size has reached 90 percent of an adult-sized brain;<ref name=Dahl_2009>{{cite journal | vauthors =Dahl RE | title = The regulation of sleep and arousal: Development and psychopathology | journal = Development and Psychopathology | year = 2009 | volume = 8 | issue =1 | pages = 3–27 | doi = 10.1017/S0954579400006945 | s2cid = 143514600 }}</ref> a majority of this brain growth has occurred during the period of life with the highest rate of sleep. The hours that children spend asleep influence their ability to perform on cognitive tasks.<ref name = "Jenni_Dahl_2008" >{{cite book |veditors=Nelson CA, Luciana M | title = Handbook of developmental cognitive neuroscience |url=https://archive.org/details/handbookdevelopm00nels |url-access=limited | year = 2008 | publisher = MIT Press | location = Cambridge, Mass. | isbn = 978-0-262-14104-8 | edition = 2nd |vauthors=Jenni OG, Dahl RE | chapter = Sleep, cognition, and neuron, and emotion: A developmental review. | pages = [https://archive.org/details/handbookdevelopm00nels/page/n819 807]–817 }}</ref><ref name="Scher_2005">{{cite journal | vauthors = Scher A | title = Infant sleep at 10 months of age as a window to cognitive development | journal = Early Human Development | volume = 81 | issue = 3 | pages = 289–92 | date = March 2005 | pmid = 15814211 | doi = 10.1016/j.earlhumdev.2004.07.005 }}</ref> Children who sleep through the night and have few night waking episodes have higher cognitive attainments and easier temperaments than other children.<ref name="Scher_2005"/><ref name="Spruyt_2007">{{cite journal | vauthors = Spruyt K, Aitken RJ, So K, Charlton M, Adamson TM, Horne RS | title = Relationship between sleep/wake patterns, temperament and overall development in term infants over the first year of life | journal = Early Human Development | volume = 84 | issue = 5 | pages = 289–96 | date = May 2008 | pmid = 17707119 | doi = 10.1016/j.earlhumdev.2007.07.002 }}</ref><ref name="Bernier_2010">{{cite journal | vauthors = Bernier A, Carlson SM, Bordeleau S, Carrier J | title = Relations between physiological and cognitive regulatory systems: infant sleep regulation and subsequent executive functioning | journal = Child Development | volume = 81 | issue = 6 | pages = 1739–52 | year = 2010 | pmid = 21077861 | doi = 10.1111/j.1467-8624.2010.01507.x | doi-access = free }}</ref>

Sleep also influences language development. To test this, researchers taught infants a faux language and observed their recollection of the rules for that language.<ref name="Hupbach_2009">{{cite journal | vauthors = Hupbach A, Gomez RL, Bootzin RR, Nadel L | title = Nap-dependent learning in infants | journal = Developmental Science | volume = 12 | issue = 6 | pages = 1007–12 | date = November 2009 | pmid = 19840054 | doi = 10.1111/j.1467-7687.2009.00837.x | citeseerx = 10.1.1.712.685 }}</ref> Infants who slept within four hours of learning the language could remember the language rules better, while infants who stayed awake longer did not recall those rules as well. There is also a relationship between infants' vocabulary and sleeping: infants who sleep longer at night at 12 months have better vocabularies at 26 months.<ref name="Bernier_2010"/>

Children can greatly benefit from a structured bedtime routine. This can look differently among families, but will generally consist of a set of rituals such as reading a bedtime story, a bath, brushing teeth, and can also include a show of affection from the parent to the child such as a hug or kiss before bed. A bedtime routine will also include a consistent time that the child is expected to be in bed ready for sleep. Having a reliable bedtime routine can help improve a child's quality of sleep as well as prepare them to make and keep healthy [[sleep hygiene]] habits in the future.<ref>{{cite journal | vauthors = Mindell JA, Williamson AA | title = Benefits of a bedtime routine in young children: Sleep, development, and beyond | journal = Sleep Medicine Reviews | volume = 40 | pages = 93–108 | date = August 2018 | pmid = 29195725 | pmc = 6587181 | doi = 10.1016/j.smrv.2017.10.007 }}</ref>

=== Recommended duration ===
[[File:"Plenty of sleep keeps him on the job" - NARA - 514792.jpg|thumb|World War II poster issued by the US government]]Children need many hours of sleep per day in order to develop and function properly: up to 18 hours for [[newborn]] babies, with a declining rate as a child ages.<ref name="autogenerated1"/> Early in 2015, after a two-year study,<ref name="sleepfoundation">{{cite journal | vauthors = Hirshkowitz M, Whiton K, Albert SM, Alessi C, Bruni O, DonCarlos L, Hazen N, Herman J, Katz ES, Kheirandish-Gozal L, Neubauer DN, O'Donnell AE, Ohayon M, Peever J, Rawding R, Sachdeva RC, Setters B, Vitiello MV, Ware JC, Adams Hillard PJ | display-authors = 6 | title = National Sleep Foundation's sleep time duration recommendations: methodology and results summary | journal = Sleep Health | volume = 1 | issue = 1 | pages = 40–43 | date = March 2015 | pmid = 29073412 | doi = 10.1016/j.sleh.2014.12.010 | s2cid = 205190733 | url = https://www.sleephealthjournal.org/article/S2352-7218(15)00015-7/fulltext | access-date = 4 February 2015 | url-status = live | archive-url = https://web.archive.org/web/20171114014645/https://www.sleephealthjournal.org/article/S2352-7218(15)00015-7/fulltext | archive-date = 14 November 2017 | url-access = subscription }}</ref> the [[National Sleep Foundation]] in the US announced newly revised recommendations as shown in the table below.

{| class="wikitable"
|+Hours of sleep recommended for each age group<ref name="sleepfoundation" />
|-
! Age and condition
! Sleep needs
|-
| Newborns (0–3 months)
| 14 to 17 hours
|-
| Infants (4–11 months)
| 12 to 15 hours
|-
| Toddlers (1–2 years)
| 11 to 14 hours
|-
| Preschoolers (3–4 years)
| 10 to 13 hours
|-
| School-age children (5–12 years)&nbsp;&nbsp;&nbsp;&nbsp;
| 9 to 11 hours
|-
| Teenagers (13–17 years)
| 8 to 10 hours
|-
| Adults (18–64 years)
| 7 to 9 hours
|-
| Older Adults (65 years and over)
| 7 to 8 hours
|}

== Functions ==
{{Unsolved|biology|What is the biological function of sleep?}}

===Restoration===
{{Further|Glymphatic system}}

The sleeping brain has been shown to remove metabolic end products at a faster rate than during an awake state, by increasing the flow of cerebrospinal fluid during sleep.<ref name="Xie2013">{{cite journal |display-authors=6 |vauthors=Xie L, Kang H, Xu Q, Chen MJ, Liao Y, Thiyagarajan M, O'Donnell J, Christensen DJ, Nicholson C, Iliff JJ, Takano T, Deane R, Nedergaard M |date=17 October 2013 |title=Sleep drives metabolite clearance from the adult brain |journal=Science |volume=342 |issue=6156 |pages=373–7 |bibcode=2013Sci...342..373X |doi=10.1126/science.1241224 |issn=0036-8075 |pmc=3880190 |pmid=24136970}}</ref> The mechanism for this removal appears to be the [[glymphatic system]], a system that does for the brain what the lymphatic system does for the body.<ref name="Xie2013" /><ref name="ntyimes"/> Further research has shown that the glymphatic system is driven by pulses of hormones that in turn create surges in blood flow that cause the cerebrospinal fluid to flow, carrying away metabolites.<ref name="science1">{{cite news|last=Leslie|first=Mitch|title=Scientists uncover how the brain washes itself during sleep|url=https://www.science.org/content/article/scientists-uncover-how-brain-washes-itself-during-sleep?utm_source=Live+Audience&utm_campaign=b50db22312-nature-briefing-daily-20250108_COPY_01&utm_medium=email&utm_term=0_b27a691814-b50db22312-51308676|newspaper=[[Science (journal)|Science]]|access-date=10 January 2025|date=8 January 2025|quote=She called it the glymphatic system, a nod to its dependence on glial cells}}</ref>

Sleep may facilitate the synthesis of molecules that help repair and protect the brain from metabolic end products generated during waking.<ref>{{cite journal |vauthors=Siegel JM |date=October 2005 |title=Clues to the functions of mammalian sleep |journal=Nature |volume=437 |issue=7063 |pages=1264–71 |bibcode=2005Natur.437.1264S |doi=10.1038/nature04285 |pmc=8760626 |pmid=16251951 |s2cid=234089}}</ref> [[Anabolic]] hormones, such as [[growth hormone]]s, are secreted preferentially during sleep. The brain concentration of [[glycogen]] increases during sleep, and is depleted through metabolism during wakefulness.<ref name="CespuglioEtAl2005" />

The human organism physically restores itself during sleep, occurring mostly during [[slow-wave sleep]] during which body temperature, heart rate, and brain oxygen consumption decrease. In both the brain and body, the reduced rate of [[metabolism]] enables countervailing restorative processes.<ref name=CespuglioEtAl2005>Raymond Cespuglio, Damien Colas, & Sabine Gautier-Sauvigné, "Energy Processes Underlying the Sleep Wake Cycle"; Chapter 1 in Parmeggiani & Velluti (2005).</ref> While the body benefits from sleep, the brain actually requires sleep for restoration, whereas these processes can take place during quiescent waking in the rest of the body.<ref>{{cite journal | vauthors = Eugene AR, Masiak J | title = The Neuroprotective Aspects of Sleep | journal = MEDtube Science | volume = 3 | issue = 1 | pages = 35–40 | date = March 2015 | pmid = 26594659 | pmc = 4651462 }}</ref> The essential function of sleep may be its restorative effect on the brain: "Sleep is of the brain, by the brain and for the brain."<ref name=Hobson2005>{{cite journal | vauthors = Hobson JA | title = Sleep is of the brain, by the brain and for the brain | journal = Nature | volume = 437 | issue = 7063 | pages = 1254–1256 | date = October 2005 | pmid = 16251949 | doi = 10.1038/nature04283 | s2cid = 1055112 | bibcode = 2005Natur.437.1254H }}</ref> Furthermore, this includes almost any brain, no matter how small: sleep is observed to be a necessary behavior across most of the animal kingdom, including some of the least cognitively advanced animals, implying that sleep is essential to the most fundamental brain processes, i.e. neuronal firing.  This shows that sleep is vital even when there is no need for other functions of sleep, such as [[memory consolidation]] or dreaming.<ref name=Joiner2016/>

=== Memory processing ===
{{Further|Sleep and memory|Neuroscience of sleep|Sleep and learning|Page 4=Neuroscience of sleep}}

It has been widely accepted that sleep must support the formation of long-term memory, and generally increasing previous learning and experiences recalls. However, its benefit seems to depend on the phase of sleep and the type of memory.<ref>{{cite journal | vauthors = Plihal W, Born J | title = Effects of early and late nocturnal sleep on declarative and procedural memory | journal = Journal of Cognitive Neuroscience | volume = 9 | issue = 4 | pages = 534–47 | date = July 1997 | pmid = 23968216 | doi = 10.1162/jocn.1997.9.4.534 | s2cid = 3300300 }}</ref> For example, declarative and procedural memory-recall tasks applied over early and late nocturnal sleep, as well as wakefulness controlled conditions, have been shown that declarative memory improves more during early sleep (dominated by SWS) while procedural memory during late sleep (dominated by REM sleep) does so.<ref name=":02">{{cite journal | vauthors = Rasch B, Büchel C, Gais S, Born J | title = Odor cues during slow-wave sleep prompt declarative memory consolidation | journal = Science | volume = 315 | issue = 5817 | pages = 1426–9 | date = March 2007 | pmid = 17347444 | doi = 10.1126/science.1138581 | s2cid = 19788434 | bibcode = 2007Sci...315.1426R }}</ref><ref name=":13">{{cite journal | vauthors = Born J, Wilhelm I | title = System consolidation of memory during sleep | journal = Psychological Research | volume = 76 | issue = 2 | pages = 192–203 | date = March 2012 | pmid = 21541757 | pmc = 3278619 | doi = 10.1007/s00426-011-0335-6 }}</ref>

With regard to declarative memory, the functional role of SWS has been associated with hippocampal replays of previously encoded neural patterns that seem to facilitate long-term memory consolidation.<ref name=":02" /><ref name=":13" /> This assumption is based on the active system consolidation hypothesis, which states that repeated reactivations of newly encoded information in the hippocampus during slow oscillations in NREM sleep mediate the stabilization and gradual integration of declarative memory with pre-existing knowledge networks on the cortical level.<ref>{{cite journal | vauthors = Diekelmann S, Born J | title = The memory function of sleep | journal = Nature Reviews. Neuroscience | volume = 11 | issue = 2 | pages = 114–26 | date = February 2010 | pmid = 20046194 | doi = 10.1038/nrn2762 | s2cid = 1851910 }}</ref> It assumes the hippocampus might hold information only temporarily and in a fast-learning rate, whereas the neocortex is related to long-term storage and a slow-learning rate.<ref name=":02" /><ref name=":13" /><ref name=":1">{{cite journal | vauthors = Rasch B, Born J | title = About sleep's role in memory | journal = Physiological Reviews | volume = 93 | issue = 2 | pages = 681–766 | date = April 2013 | pmid = 23589831 | pmc = 3768102 | doi = 10.1152/physrev.00032.2012 }}</ref><ref name=":22">{{cite journal | vauthors = Schreiner T, Rasch B | title = Boosting Vocabulary Learning by Verbal Cueing During Sleep | journal = Cerebral Cortex | volume = 25 | issue = 11 | pages = 4169–79 | date = November 2015 | pmid = 24962994 | doi = 10.1093/cercor/bhu139 | doi-access = free }}</ref><ref name=":52">{{cite journal | vauthors = Schreiner T, Rasch B | title = The beneficial role of memory reactivation for language learning during sleep: A review | journal = Brain and Language | volume = 167 | pages = 94–105 | date = April 2017 | pmid = 27036946 | doi = 10.1016/j.bandl.2016.02.005 | url = https://doc.rero.ch/record/323249/files/schreinerrasch2017_brainlang.pdf | s2cid = 3377186 }}</ref> This dialogue between the hippocampus and neocortex occurs in parallel with hippocampal [[Sharp waves and ripples|sharp-wave ripples]] and [[Sleep spindle|thalamo-cortical spindles]], synchrony that drives the formation of the spindle-ripple event which seems to be a prerequisite for the formation of long-term memories.<ref name=":13" /><ref name=":1" /><ref name=":52" /><ref name=":32">{{cite journal | vauthors = Ngo HV, Martinetz T, Born J, Mölle M | title = Auditory closed-loop stimulation of the sleep slow oscillation enhances memory | journal = Neuron | volume = 78 | issue = 3 | pages = 545–53 | date = May 2013 | pmid = 23583623 | doi = 10.1016/j.neuron.2013.03.006 | doi-access = free }}</ref>

Reactivation of memory also occurs during wakefulness and its function is associated with serving to update the reactivated memory with newly encoded information, whereas reactivations during SWS are presented as crucial for memory stabilization.<ref name=":13" /> Based on targeted memory reactivation (TMR) experiments that use associated memory cues to triggering memory traces during sleep, several studies have been reassuring the importance of nocturnal reactivations for the formation of persistent memories in neocortical networks, as well as highlighting the possibility of increasing people's memory performance at declarative recalls.<ref name=":02" /><ref name=":22" /><ref name=":52" /><ref name=":32" /><ref>{{cite journal | vauthors = Klinzing JG, Kugler S, Soekadar SR, Rasch B, Born J, Diekelmann S | title = Odor cueing during slow-wave sleep benefits memory independently of low cholinergic tone | journal = Psychopharmacology | volume = 235 | issue = 1 | pages = 291–299 | date = January 2018 | pmid = 29119218 | pmc = 5748395 | doi = 10.1007/s00213-017-4768-5 }}</ref>

Furthermore, nocturnal reactivation seems to share the same neural oscillatory patterns as reactivation during wakefulness, processes which might be coordinated by [[Theta wave|theta activity]].<ref name=":4">{{cite journal | vauthors = Schreiner T, Doeller CF, Jensen O, Rasch B, Staudigl T | title = Theta Phase-Coordinated Memory Reactivation Reoccurs in a Slow-Oscillatory Rhythm during NREM Sleep | journal = Cell Reports | volume = 25 | issue = 2 | pages = 296–301 | date = October 2018 | pmid = 30304670 | pmc = 6198287 | doi = 10.1016/j.celrep.2018.09.037 }}</ref> During wakefulness, theta oscillations have been often related to successful performance in memory tasks, and cued memory reactivations during sleep have been showing that theta activity is significantly stronger in subsequent recognition of cued stimuli as compared to uncued ones, possibly indicating a strengthening of memory traces and lexical integration by cuing during sleep.<ref>{{cite journal | vauthors = Schreiner T, Göldi M, Rasch B | title = Cueing vocabulary during sleep increases theta activity during later recognition testing | journal = Psychophysiology | volume = 52 | issue = 11 | pages = 1538–43 | date = November 2015 | pmid = 26235609 | doi = 10.1111/psyp.12505 }}</ref> However, the beneficial effect of TMR for memory consolidation seems to occur only if the cued memories can be related to prior knowledge.<ref>{{cite journal | vauthors = Groch S, Schreiner T, Rasch B, Huber R, Wilhelm I | title = Prior knowledge is essential for the beneficial effect of targeted memory reactivation during sleep | journal = Scientific Reports | volume = 7 | article-number = 39763 | date = January 2017 | pmid = 28051138 | pmc = 5209656 | doi = 10.1038/srep39763 | bibcode = 2017NatSR...739763G | doi-access = free }}</ref>

=== Dreaming ===
{{Main|Dream}}
[[File:Glimpse of a dream (9391068364).jpg|thumb|Dreams often feel like waking life, yet with added surrealism.]]During sleep, especially REM sleep, humans tend to experience dreams. These are elusive and mostly unpredictable first-person experiences which seem logical and realistic to the dreamer while they are in progress, despite their frequently bizarre, irrational, and/or surreal qualities that become apparent when assessed after waking. Dreams often seamlessly incorporate concepts, situations, people, and objects within a person's mind that would not normally go together. They can include apparent sensations of all types, especially vision and movement.<ref name=HobsonEtAl2000>J. Alan Hobson, Edward F. Pace-Scott, & Robert Stickgold (2000), "Dreaming and the brain: Toward a cognitive neuroscience of conscious states", ''Behavioral and Brain Sciences'' 23.</ref>

Dreams tend to rapidly fade from memory after waking. Some people choose to keep a [[dream journal]], which they believe helps them build dream recall and facilitate the ability to experience [[lucid dreams]].

A lucid dream is a type of dream in which the dreamer becomes aware that they are dreaming while dreaming. In a preliminary study, dreamers were able to consciously [[Lucid dream#Two-way communication|communicate]] with experimenters via eye movements or facial muscle signals, and were able to comprehend complex questions and use working memory.<ref name="10.1016/j.cub.2021.01.026">{{cite journal|display-authors=3 |last1=Konkoly |first1=Karen R. |last2=Appel |first2=Kristoffer |last3=Chabani |first3=Emma |last4=Mangiaruga |first4=Anastasia |last5=Gott |first5=Jarrod |last6=Mallett |first6=Remington |last7=Caughran |first7=Bruce |last8=Witkowski |first8=Sarah |last9=Whitmore |first9=Nathan W. |last10=Mazurek |first10=Christopher Y. |last11=Berent |first11=Jonathan B. |last12=Weber |first12=Frederik D. |last13=Türker |first13=Başak |last14=Leu-Semenescu |first14=Smaranda |last15=Maranci |first15=Jean-Baptiste |last16=Pipa |first16=Gordon |last17=Arnulf |first17=Isabelle |last18=Oudiette |first18=Delphine |last19=Dresler |first19=Martin |last20=Paller |first20=Ken A. |title=Real-time dialogue between experimenters and dreamers during REM sleep |journal=Current Biology |date=18 February 2021 |volume=31 |issue=7 |pages=1417–1427.e6 |doi=10.1016/j.cub.2021.01.026 |pmid=33607035 |pmc=8162929 |language=English |issn=0960-9822|doi-access=free |bibcode=2021CBio...31E1417K }} [[File:CC-BY icon.svg|50px]] Available under [https://creativecommons.org/licenses/by/4.0/ CC BY 4.0] {{Webarchive|url=https://web.archive.org/web/20171016050101/https://creativecommons.org/licenses/by/4.0/ |date=16 October 2017 }}.</ref>

People have proposed many [[hypotheses]] about the functions of dreaming. [[Sigmund Freud]] postulated that dreams are the symbolic expression of frustrated desires that have been relegated to the [[unconscious mind]], and he used [[dream interpretation]] in the form of [[psychoanalysis]] in attempting to uncover these desires.<ref>See Freud: ''[[The Interpretation of Dreams]]''.</ref>

Counterintuitively, [[nocturnal penile tumescence|penile erections during sleep]] are not more frequent during sexual dreams than during other dreams.<ref>{{cite book| vauthors = Pinel JP |title=Biopsychology, 8th Edition|year=2011|publisher=Pearson Education, Inc.|isbn=978-0-205-83256-9|page=359}}</ref> The [[parasympathetic nervous system]] experiences increased activity during REM sleep which may cause erection of the penis or clitoris. In males, 80% to 95% of REM sleep is normally accompanied by partial to full penile erection, while only about 12% of men's dreams contain sexual content.<ref name="Saladin 2012 537">{{cite book| vauthors = Saladin KS |title=Anatomy and Physiology: The Unity of Form and Function|edition=6th|year=2012|publisher=McGraw-Hill|isbn=978-0-07-337825-1|page=537}}</ref>

==Disorders==
===Insomnia===
{{main|Insomnia}} {{See also|Psychological stress and sleep}}
Insomnia is a general term for difficulty falling asleep and/or staying asleep. Insomnia is the most common sleep problem, with many adults reporting occasional insomnia, and 10–15% reporting a chronic condition.<ref>[[#Brown|Brown]], pp. 1146–1147.</ref> Insomnia can have many different causes, including [[Psychological stress and Sleep|psychological stress]], a poor sleep environment, an inconsistent sleep schedule, or excessive mental or physical stimulation in the hours before bedtime. Insomnia is often treated through behavioral changes like keeping a regular sleep schedule, avoiding stimulating or stressful activities before bedtime, and cutting down on stimulants such as caffeine. The sleep environment may be improved by installing heavy drapes to shut out all sunlight, and keeping computers, televisions, and work materials out of the sleeping area.

A 2010 review of published scientific research suggested that exercise generally improves sleep for most people, and helps sleep disorders such as insomnia. The optimum time to exercise ''may'' be 4 to 8 hours before bedtime, though exercise at any time of day is beneficial, with the exception of heavy exercise taken shortly before bedtime, which may disturb sleep. However, there is insufficient evidence to draw detailed conclusions about the relationship between exercise and sleep.<ref>{{cite journal | vauthors = Buman MP, King AC |title=Exercise as a Treatment to Enhance Sleep |journal=American Journal of Lifestyle Medicine |volume=4|issue=6|pages=500–514|doi=10.1177/1559827610375532|year=2010|s2cid=73314918 }}</ref> [[Nonbenzodiazepine]] sleeping medications such as [[Zolpidem|Ambien]], [[Zopiclone|Imovane]], and [[Eszopiclone|Lunesta]] (also known as "Z-drugs"), while initially believed to be entirely better and safer than earlier generations of {{nowrap|sedatives{{tsp}}{{mdash}}}}{{tsp}}in{{shy}}clud{{shy}}ing [[benzodiazepine]] {{nowrap|drugs{{tsp}}{{mdash}}}}{{tsp}}are now known to be similar in more ways than thought. [[White noise]] appears to be a promising treatment for [[insomnia]].<ref>{{cite journal | vauthors = López HH, Bracha AS, Bracha HS | title = Evidence based complementary intervention for insomnia | journal = Hawaii Medical Journal | volume = 61 | issue = 9 | pages = 192, 213 | date = September 2002 | pmid = 12422383 | url = https://cogprints.org/5032/1/2002_H.M.J_White-noise_for_PTSD.pdf | access-date = 16 December 2010 | url-status = live | archive-url = https://web.archive.org/web/20150501085132/https://cogprints.org/5032/1/2002_H.M.J_White-noise_for_PTSD.pdf | archive-date = 1 May 2015 }}</ref>

==Sleep health==
{{see also|Sleep hygiene}}

===Sleep duration and quality===
Sleep duration measures the length of sleep, whereas sleep quality includes factors such as speed in falling asleep and whether sleep is unbroken.<ref>{{cite web | url=https://www.sleepfoundation.org/sleep-hygiene/how-to-determine-poor-quality-sleep | title=How to Determine Poor Sleep Quality | date=17 December 2020 }}</ref><ref>{{cite web | url=https://www.thensf.org/what-is-sleep-quality/ | title=What is Sleep Quality? | date=12 April 2024 }}</ref> Adequate quality sleep is linked with better mood and the abilities to express and quickly process [[emotion]].<ref>{{Cite journal |last1=Lollies |first1=Friederike |last2=Schnatschmidt |first2=Marisa |last3=Bihlmeier |first3=Isabell |last4=Genuneit |first4=Jon |last5=In-Albnon |first5=Tina |last6=Holtmann |first6=Martin |last7=Legenbauer |first7=Tanja |last8=Schlarb |first8=Angelika Anita |date=December 2022 |title=Associations of sleep and emotion regulation processes in childhood and adolescence - a systematic review, report of methodological challenges and future directions |journal=Sleep Science |language=en |volume=15 |issue=4 |pages=490–514 |doi=10.5935/1984-0063.20220082 |pmid=36419813 |issn=1984-0659 |pmc=9670771 }}</ref>

Low quality sleep has been linked with health conditions like [[cardiovascular disease]], [[obesity]], and [[mental illness]]. While poor sleep is common among those with cardiovascular disease, some research indicates that poor sleep can be a contributing cause. Short sleep duration of less than seven hours is correlated with [[coronary heart disease]] and increased risk of death from coronary heart disease. Sleep duration greater than nine hours is also correlated with coronary heart disease, as well as [[stroke]] and cardiovascular events.<ref name="Hale">{{cite journal | vauthors = Hale L, Troxel W, Buysse DJ | title = Sleep Health: An Opportunity for Public Health to Address Health Equity | journal = Annual Review of Public Health | volume = 41 | issue = 1 | pages = 81–99 | date = April 2020 | pmid = 31900098 | pmc = 7944938 | doi = 10.1146/annurev-publhealth-040119-094412 | doi-access = free }}</ref><ref name="Jackson">{{cite journal | vauthors = Jackson CL, Redline S, Emmons KM | title = Sleep as a potential fundamental contributor to disparities in cardiovascular health | journal = Annual Review of Public Health | volume = 36 | issue = 1 | pages = 417–440 | date = March 2015 | pmid = 25785893 | pmc = 4736723 | doi = 10.1146/annurev-publhealth-031914-122838 }}</ref><ref name="Cespedes">{{cite journal | vauthors = Cespedes Feliciano EM, Quante M, Rifas-Shiman SL, Redline S, Oken E, Taveras EM | title = Objective Sleep Characteristics and Cardiometabolic Health in Young Adolescents | journal = Pediatrics | volume = 142 | issue = 1 | pages = e20174085 | date = July 2018 | pmid = 29907703 | pmc = 6260972 | doi = 10.1542/peds.2017-4085 }}</ref><ref name="pmid27647451">{{cite journal | vauthors = St-Onge MP, Grandner MA, Brown D, Conroy MB, Jean-Louis G, Coons M, Bhatt DL | title = Sleep Duration and Quality: Impact on Lifestyle Behaviors and Cardiometabolic Health: A Scientific Statement From the American Heart Association | journal = Circulation | volume = 134 | issue = 18 | pages = e367–e386 | date = November 2016 | pmid = 27647451 | pmc = 5567876 | doi = 10.1161/CIR.0000000000000444 | type = Review }}</ref>

In both children and adults, short sleep duration is associated with an increased risk of obesity, with various studies reporting an increased risk of 45–55%. Other aspects of sleep health have been associated with obesity, including daytime napping, sleep timing, the variability of sleep timing, and low sleep efficiency. However, sleep duration is the most-studied for its impact on obesity.<ref name="Hale"/>

Sleep problems have been frequently viewed as a symptom of mental illness rather than a causative factor. However, a growing body of evidence suggests that they are both a cause and a symptom of mental illness. Insomnia is a significant predictor of [[major depressive disorder]]; a [[meta-analysis]] of 170,000 people showed that insomnia at the beginning of a study period indicated a more than the twofold increased risk for major depressive disorder. Some studies have also indicated correlation between insomnia and [[anxiety]], [[post-traumatic stress disorder]], and [[suicide]]. Sleep disorders can increase the risk of [[psychosis]] and worsen the severity of psychotic episodes.<ref name="Hale"/>

Sleep research also displays [[race and health|differences in race]] and class. Short sleep and poor sleep are observed more frequently in ethnic minorities than in whites in the US. African-Americans report experiencing short durations of sleep five times more often than whites, possibly as a result of social and environmental factors. A study done in the USA suggested that higher rates of sleep apnea (and poorer responses to treatment) are suffered by children in disadvantaged neighborhoods (which, in context, includes a disproportionate effect on children of African-American descent).<ref name="Wang">{{cite journal | vauthors = Wang R, Dong Y, Weng J, Kontos EZ, Chervin RD, Rosen CL, Marcus CL, Redline S | display-authors = 6 | title = Associations among Neighborhood, Race, and Sleep Apnea Severity in Children. A Six-City Analysis | journal = Annals of the American Thoracic Society | volume = 14 | issue = 1 | pages = 76–84 | date = January 2017 | pmid = 27768852 | pmc = 5291481 | doi = 10.1513/AnnalsATS.201609-662OC }}</ref>

===Sleep hygiene===
Sleep health can be improved through implementing good [[sleep hygiene]] habits. Having good sleep hygiene can help to improve your physical and mental health by providing your body with the necessary rejuvenation only restful sleep can provide.<ref>{{Cite web|date=18 September 2020|title=Mental Health and Sleep|url=https://www.sleepfoundation.org/mental-health|access-date=18 November 2021|website=Sleep Foundation|language=en}}</ref> Some ways to improve sleep health include going to sleep at consistent times every night, avoiding any electronic devices such as televisions in the bedroom, getting adequate exercise throughout your day, and avoiding caffeine in the hours before going to sleep. Another way to greatly improve sleep hygiene is by creating a peaceful and relaxing sleep environment. Sleeping in a dark and clean room with things like a white noise maker can help facilitate restful sleep.<ref>{{Cite web|date=13 February 2019|title=CDC - Sleep Hygiene Tips - Sleep and Sleep Disorders|url=https://www.cdc.gov/sleep/about_sleep/sleep_hygiene.html|access-date=18 November 2021|website=cdc.gov|language=en-us}}</ref> However, [[noise]], with the exception of [[white noise]], may not be good for sleep.

==Drugs and diet==
{{See also|Alertness#Drugs used to increase alertness}}
Drugs which induce sleep, known as [[hypnotic]]s, include [[benzodiazepine]]s (although these interfere with REM);<ref name="sleep_medicine_a04">{{Cite book | vauthors = Lee-chiong T |title=Sleep Medicine: Essentials and Review |year= 2008 |publisher=[[Oxford University Press]], US |url=https://books.google.com/books?id=s1F_DEbRNMcC&pg=PT52 |isbn=978-0-19-530659-0 |page=52 |access-date=25 September 2016 |archive-url=https://web.archive.org/web/20170311005733/https://books.google.com/books?id=s1F_DEbRNMcC&pg=PT52 |archive-date=11 March 2017 |url-status=live }}</ref> [[nonbenzodiazepine]] hypnotics such as [[eszopiclone]] (Lunesta), [[zaleplon]] (Sonata), and [[zolpidem]] (Ambien); [[antihistamine]]s such as [[diphenhydramine]] (Benadryl) and [[doxylamine]]; [[Alcohol (drug)|alcohol (ethanol)]], (which exerts an excitatory [[rebound effect]] later in the night and intereferes with REM)<ref name="sleep_medicine_a04"/> [[barbiturate]]s (which have the same problem), [[melatonin (medication)|melatonin]] (a component of the circadian clock)<ref name=TurekCzeisler1999>{{cite book | vauthors = Turek FW, Czeisler CA | date = 1999 | chapter = Role of Melatonin in the Regulation of Sleep | veditors = Zee PC, Turek FW | title = Regulation of Sleep and Circadian Rhythms | pages = 181–195 }}</ref> and [[cannabis (drug)|cannabis]] (which may also interfere with REM).<ref>[https://www.psychologytoday.com/blog/the-teenage-mind/200906/marijuana-sleep-and-dreams Marijuana, Sleep and Dreams]. psychologytoday.com. Retrieved on 10 February 2012.</ref> Some [[opioid]]s (including [[morphine]], [[codeine]], [[heroin]], and [[oxycodone]]) also induce sleep, and can disrupt sleep architecture and sleep stage distribution.<ref>{{cite journal |last1=Rosen |first1=Ilene M. |last2=Aurora |first2=R. Nisha |last3=Kirsch |first3=Douglas B. |last4=Carden |first4=Kelly A. |last5=Malhotra |first5=Raman K. |last6=Ramar |first6=Kannan |last7=Abbasi-Feinberg |first7=Fariha |last8=Kristo |first8=David A. |last9=Martin |first9=Jennifer L. |last10=Olson |first10=Eric J. |last11=Rosen |first11=Carol L. |last12=Rowley |first12=James A. |last13=Shelgikar |first13=Anita V. |title=Chronic Opioid Therapy and Sleep: An American Academy of Sleep Medicine Position Statement |journal=Journal of Clinical Sleep Medicine |date=15 November 2019 |volume=15 |issue=11 |pages=1671–1673 |doi=10.5664/jcsm.8062 |pmid=31739858 |pmc=6853382 |language=en |issn=1550-9389}}</ref> The endogenously produced drug [[Gamma-Hydroxybutyric acid|gamma-hydroxybutyrate]] (GHB) is capable of producing high quality sleep that is indistinguishable from natural sleep architecture in humans.<ref>{{Cite journal |last1=Mamelak |first1=M. |last2=Escriu |first2=J. M. |last3=Stokan |first3=O. |date=April 1977 |title=The effects of gamma-hydroxybutyrate on sleep |journal=Biological Psychiatry |volume=12 |issue=2 |pages=273–288 |issn=0006-3223 |pmid=192353}}</ref>

[[Stimulant]]s, which inhibit sleep, include [[caffeine]], an [[adenosine]] antagonist; [[amphetamine]], [[methamphetamine]], [[MDMA]], [[empathogen-entactogen]]s, and related drugs; [[cocaine]], which can alter the circadian rhythm,<ref>{{cite journal | vauthors = Abarca C, Albrecht U, Spanagel R | title = Cocaine sensitization and reward are under the influence of circadian genes and rhythm | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 99 | issue = 13 | pages = 9026–30 | date = June 2002 | pmid = 12084940 | pmc = 124417 | doi = 10.1073/pnas.142039099 | bibcode = 2002PNAS...99.9026A | doi-access = free }}</ref><ref>[https://web.archive.org/web/20080714095916/https://www.mindsite.com/dsm_iv/primary_hypersomnia Primary hypersomnia: Diagnostic Features]. mindsite.com</ref> and [[methylphenidate]], which acts similarly; and [[eugeroic]] drugs like [[modafinil]] and [[armodafinil]] with poorly understood mechanisms. Consuming high amounts of the stimulant caffeine can result in interrupted sleep patterns and sometimes sleep deprivation. This vicious cycle can result in drowsiness which can then result in a higher consumption of caffeine in order to stay awake the next day. This cycle can lead to decreased cognitive function and an overall feeling of fatigue.<ref>{{cite journal | vauthors = O'Callaghan F, Muurlink O, Reid N | title = Effects of caffeine on sleep quality and daytime functioning | journal = Risk Management and Healthcare Policy | volume = 11 | pages = 263–271 | date = 7 December 2018 | pmid = 30573997 | pmc = 6292246 | doi = 10.2147/RMHP.S156404 | doi-access = free }}</ref>

Some drugs may alter sleep architecture without inhibiting or inducing sleep. Drugs that amplify or inhibit endocrine and immune system secretions associated with certain sleep stages have been shown to alter sleep architecture.<ref>{{Cite journal |last=Shimatsu |first=Akira |date=September 2004 |title=[Ghrelin-related drugs: clinical perspectives] |journal=Nihon Rinsho. Japanese Journal of Clinical Medicine |volume=62 |issue=Suppl 9 |pages=435–438 |issn=0047-1852 |pmid=15506422}}</ref><ref>{{Cite journal |last1=Murphy |first1=P. J. |last2=Badia |first2=P. |last3=Myers |first3=B. L. |last4=Boecker |first4=M. R. |last5=Wright |first5=K. P. |date=June 1994 |title=Nonsteroidal anti-inflammatory drugs affect normal sleep patterns in humans |journal=Physiology & Behavior |volume=55 |issue=6 |pages=1063–1066 |doi=10.1016/0031-9384(94)90388-3 |issn=0031-9384 |pmid=8047572|s2cid=25887442 }}</ref> The growth hormone releasing hormone receptor agonist [[MK-677]] has been shown to increase REM in older adults as well as stage IV sleep in younger adults by approximately 50%.<ref>{{Cite journal |last1=Copinschi |first1=G. |last2=Leproult |first2=R. |last3=Van Onderbergen |first3=A. |last4=Caufriez |first4=A. |last5=Cole |first5=K. Y. |last6=Schilling |first6=L. M. |last7=Mendel |first7=C. M. |last8=De Lepeleire |first8=I. |last9=Bolognese |first9=J. A. |last10=Van Cauter |first10=E. |date=October 1997 |title=Prolonged oral treatment with MK-677, a novel growth hormone secretagogue, improves sleep quality in man |journal=Neuroendocrinology |volume=66 |issue=4 |pages=278–286 |doi=10.1159/000127249 |issn=0028-3835 |pmid=9349662}}</ref>

===Diet===
Dietary and nutritional choices may affect sleep duration and quality. One 2016 review indicated that a high-[[carbohydrate]] diet promoted a shorter onset to sleep and a longer duration of sleep than a high-fat diet.<ref name="an">{{cite journal | vauthors = St-Onge MP, Mikic A, Pietrolungo CE | title = Effects of Diet on Sleep Quality | journal = Advances in Nutrition | volume = 7 | issue = 5 | pages = 938–49 | date = September 2016 | pmid = 27633109 | pmc = 5015038 | doi = 10.3945/an.116.012336 }}</ref> A 2012 investigation indicated that mixed [[micronutrient]]s and [[macronutrient]]s are needed to promote quality sleep.<ref>{{cite journal | vauthors = Peuhkuri K, Sihvola N, Korpela R | title = Diet promotes sleep duration and quality | journal = Nutrition Research | volume = 32 | issue = 5 | pages = 309–19 | date = May 2012 | pmid = 22652369 | doi = 10.1016/j.nutres.2012.03.009 }}</ref> A varied diet containing fresh fruits and vegetables, low [[saturated fat]], and [[whole grain]]s may be optimal for individuals seeking to improve sleep quality.<ref name=an/> [[Epidemiology|Epidemiological]] studies indicate fewer insomnia symptoms and better sleep quality with a [[Mediterranean diet]].<ref name="pmid27633109">{{cite journal | vauthors=St-Onge M, Mikic A, Cara E, Pietrolungo CE | title=Effects of Diet on Sleep Quality | journal= [[Advances in Nutrition]] | volume=7 | issue=5 | pages=938–949 | year=2016 |  doi = 10.3945/an.116.012336 | pmc=5015038 | pmid=27633109 }}</ref><ref name="pmid37156196">{{cite journal | vauthors=St-Onge M, Cherta-Murillo A, Owen L | title=The interrelationship between sleep, diet, and glucose metabolism | journal= [[Sleep Medicine Reviews]] | volume=69 | article-number=101788 | year=2023 |  doi = 10.1016/j.smrv.2023.101788 | pmc=10247426 | pmid=37156196 }}</ref> Two studies have indicated a benefit of [[Prunus cerasus|tart cherry]] juice for insomnia, or for increasing sleep efficiency as well as total sleep time.<ref name="pmid27633109" /> High-quality [[clinical trial]]s on long-term dietary practices are needed to better define the influence of diet on sleep quality.<ref name=an/>

==In culture==

===Anthropology===
[[File:Pieter Bruegel d. Ä. 037.jpg|thumb|''[[The Land of Cockaigne (Bruegel)|The Land of Cockaigne]]'' by [[Pieter Bruegel the Elder]], 1567]]

Research suggests that sleep patterns vary significantly across cultures.<ref name=Worthman>{{cite book | vauthors = Worthman CM, Melby MK | veditors = Carskadon MA |title=Adolescent Sleep Patterns: Biological, Social, and Psychological Influences |chapter=6. Toward a comparative developmental ecology of human sleep |chapter-url=https://www.cambridge.org/core/books/abs/adolescent-sleep-patterns/toward-a-comparative-developmental-ecology-of-human-sleep/96AAC229B2DE0A1E5346631BEFAEE526 |publisher=Cambridge University Press |date=2002 |pages=69–117 |doi=10.1017/CBO9780511499999.009 |isbn=978-0-521-64291-0 }}</ref><ref name="Jeon">{{cite journal | vauthors = Jeon M, Dimitriou D, Halstead EJ | title = A Systematic Review on Cross-Cultural Comparative Studies of Sleep in Young Populations: The Roles of Cultural Factors | journal = International Journal of Environmental Research and Public Health | volume = 18 | issue = 4 | page = 2005 | date = February 2021 | pmid = 33669583 | pmc = 7922907 | doi = 10.3390/ijerph18042005 | doi-access = free }}</ref><ref name="Samson">{{cite journal | vauthors = Samson DR |title=The Human Sleep Paradox: The Unexpected Sleeping Habits of Homo sapiens |journal=Annual Review of Anthropology |date=21 October 2021 |volume=50 |issue=1 |pages=259–274 |doi=10.1146/annurev-anthro-010220-075523 |s2cid=237845665 |language=en |issn=0084-6570}}</ref> The most striking differences are observed between societies that have plentiful sources of artificial light and ones that do not. The primary difference appears to be that pre-light cultures have more broken-up sleep patterns. For example, people without artificial light might go to sleep far sooner after the sun sets, but then wake up several times throughout the night, punctuating their sleep with periods of wakefulness, perhaps lasting several hours.<ref name=Worthman/> During [[pre-industrial society|pre-industrial]] [[Europe]], [[Biphasic and polyphasic sleep|biphasic (bimodal) sleeping]] was considered the norm. Sleep onset was determined not by a set bedtime, but by whether there were things to do.<ref>{{cite web |title=Humans Used to Sleep in Two Shifts, And Maybe We Should Do It Again |date=4 April 2018 |access-date=7 February 2022 | vauthors = Jackson M, Banks S |website=Science Alert |url=https://www.sciencealert.com/humans-used-to-sleep-in-two-shifts-maybe-we-should-again}}</ref>

The boundaries between sleeping and waking are blurred in these societies. Some observers believe that nighttime sleep in these societies is most often split into two main periods, the first characterized primarily by deep sleep and the second by REM sleep.<ref name=Worthman/>

Some societies display a fragmented sleep pattern in which people sleep at all times of the day and night for shorter periods. In many [[nomad]]ic or [[hunter-gatherer]] societies, people sleep on and off throughout the day or night depending on what is happening. Plentiful [[artificial light]] has been available in the industrialized West since at least the mid-19th century, and sleep patterns have changed significantly everywhere that lighting has been introduced. In general, people sleep in a more concentrated burst through the night, going to sleep much later, although this is not always the case.<ref name=Worthman/>

Historian [[A. Roger Ekirch]] thinks that the traditional pattern of "[[segmented sleep]]," as it is called, began to disappear among the urban upper class in Europe in the late 17th century and the change spread over the next 200 years; by the 1920s "the idea of a first and second sleep had receded entirely from our social consciousness."<ref>{{cite journal | vauthors = Ekirch AR | title = Sleep we have lost: pre-industrial slumber in the British Isles | journal = The American Historical Review | volume = 106 | issue = 2 | pages = 343–86 | year = 2001 | pmid = 18680884 | doi = 10.2307/2651611 | jstor = 2651611 }}</ref><ref name=bbc>{{cite news| vauthors = Hegarty S |title=The myth of the eight-hour sleep|url=https://www.bbc.co.uk/news/magazine-16964783|access-date=22 February 2012|newspaper=BBC News|date=22 February 2012|archive-url=https://web.archive.org/web/20120222172348/https://www.bbc.co.uk/news/magazine-16964783|archive-date=22 February 2012|url-status=live}}</ref> Ekirch attributes the change to increases in "street lighting, domestic lighting and a surge in coffee houses," which slowly made nighttime a legitimate time for activity, decreasing the time available for rest.<ref name=bbc/> Today in most societies people sleep during the night, but in very hot climates they may sleep during the day.<ref>[[Ellsworth Huntington|Huntington, Ellsworth]] (1915) [https://archive.org/details/civilizationand01huntgoog ''Civilization and Climate''] {{Webarchive|url=https://web.archive.org/web/20160817211746/https://archive.org/details/civilizationand01huntgoog |date=17 August 2016 }}. Yale University Press. p. 126</ref> During [[Ramadan]], many Muslims sleep during the day rather than at night.<ref>{{cite book |title=The American Muslim Teenager's Handbook | vauthors = Hafiz D, Hafiz I, Hafiz Y |year=2009 | publisher = Simon & Schuster Children's |isbn=978-1-4169-8699-7 |url-access=registration |url=https://archive.org/details/muslimteenagersh0000hafi }}</ref>

In some societies, people sleep with at least one other person (sometimes many) or with animals. In other cultures, people rarely sleep with anyone except for an intimate partner. In almost all societies, sleeping partners are strongly regulated by social standards. For example, a person might only sleep with the [[immediate family]], the [[extended family]], a spouse or romantic partner, children, children of a certain age, children of a specific gender, peers of a certain gender, friends, peers of equal social rank, or with no one at all. Sleep may be an actively social time, depending on the sleep groupings, with no constraints on noise or activity.<ref name=Worthman/>

People sleep in a variety of locations. Some sleep directly on the ground; others on a skin or blanket; others sleep on platforms or [[bed]]s. Some sleep with blankets, some with pillows, some with simple headrests, some with no head support. These choices are shaped by a variety of factors, such as climate, protection from predators, housing type, technology, personal preference, and the incidence of pests.<ref name="Worthman"/>

===In mythology and literature===
[[File:Seven sleepers (Menologion of Basil II).jpg|thumb|upright=1.3|Medieval [[illuminated manuscript|manuscript illumination]] from the ''[[Menologion of Basil II]]'' (985 AD), showing the [[Seven Sleepers|Seven Sleepers of Ephesus]] sleeping in their cave]]
Sleep has been seen in culture as similar to death since antiquity;<ref name="William">{{cite book| vauthors = William SJ |title=Sleep and Society: Sociological Ventures into the Un(known)|date=2005|publisher=Routledge|location=New York City and London|isbn=978-0-415-35419-6|pages=95–96|url=https://books.google.com/books?id=VYkqBgAAQBAJ&q=Sleep+and+his+Brother+Death&pg=PA96}}</ref> in [[Greek mythology]], [[Hypnos]] (the god of sleep) and [[Thanatos]] (the god of death) were both said to be the children of [[Nyx]] (the goddess of night).<ref name="William" /> [[John Donne]], [[Samuel Taylor Coleridge]], [[Percy Bysshe Shelley]], John Keats and other poets have all written poems about the relationship between sleep and death.<ref name="William" /> Shelley describes them as "both so passing, strange and wonderful!"<ref name="William" /> Keats similarly poses the question: "Can death be sleep, when life is but a dream".<ref>{{Cite web|url=https://www.poemhunter.com/poem/on-death-13/|title=On Death - On Death Poem by John Keats|date=29 March 2010|website=Poem Hunter}}</ref> Many people consider dying in one's sleep is the most peaceful way to die.<ref name="William" /> Phrases such as "big sleep" and "[[rest in peace]]" are often used in reference to death,<ref name="William" /> possibly in an effort to lessen its finality.<ref name="William" /> Sleep and dreaming have sometimes been seen as providing the potential for visionary experiences. In medieval Irish tradition, in order to become a [[filí]], the poet was required to undergo a ritual called the ''[[imbas forosnai]]'', in which they would enter a mantic, trancelike sleep.<ref name="Chadwick">{{cite journal | vauthors = Chadwick NK |author-link=Nora K. Chadwick|title=Imbas Forosnai |journal=Scottish Gaelic Studies |date= 1935|volume=4 |pages=97–135 }}</ref><ref name="MacKillop">{{cite book| vauthors = MacKillop J |title=A Dictionary of Celtic Mythology|date=1998|publisher=Oxford University Press|location=Oxford|isbn=0-19-280120-1}}</ref>

Many cultural stories have been told about people falling asleep for extended periods of time.<ref name="Hansen">{{cite book| vauthors = Hansen W |title=The Book of Greek & Roman Folktales, Legends & Myths|url=https://books.google.com/books?id=WwZpDQAAQBAJ&q=long+sleep+Epimenides+Rip+Van+Winkle+Seven+Sleepers&pg=PA133|date=2017|publisher=Princeton University Press|location=Princeton, New Jersey|isbn=978-0-691-17015-2|pages=132–133}}</ref><ref name="Burstein">{{cite book| vauthors = Burstein A |title=The Original Knickerbocker: The Life of Washington Irving|url=https://archive.org/details/originalknickerb00burs|url-access=registration|quote=Rip Van Winkle.|location=New York|publisher= Basic Books|date=2007|pages=[https://archive.org/details/originalknickerb00burs/page/120 120]–338|isbn=978-0-465-00853-7}}</ref> The earliest of these stories is the ancient Greek legend of [[Epimenides|Epimenides of Knossos]].<ref name="Hansen" /><ref>{{cite book| vauthors = Welch D |title=The Theater|date=9 May 1887|publisher=Theatre Publishing Company|location=New York City, New York|page=139|url=https://books.google.com/books?id=8-QRAAAAYAAJ&q=ancient+Greek+Rip+Van+Winkle&pg=PA139|volume=3|access-date=21 June 2017}}</ref><ref name="Saint Rip">{{cite web| vauthors = Thorn J | title=Saint Rip|url=https://www.nyfolklore.org/pubs/voic36-1-2/st-rip.html|website=nyfolklore.org|publisher=Voices: The Journal of New York Folklore|access-date=21 June 2017|archive-url=https://web.archive.org/web/20171018210936/https://www.nyfolklore.org/pubs/voic36-1-2/st-rip.html|archive-date=18 October 2017|url-status=live}}</ref><ref>{{cite book| vauthors = Bates A |title=The Drama; Its History, Literature and Influence on Civilization: American Drama|date=1906|publisher=Historical Publishing Company|location=London, England|page=121|url=https://books.google.com/books?id=Nx8ZAQAAIAAJ&q=ancient+Greek+Rip+Van+Winkle&pg=PA121|volume=20|access-date=21 June 2017}}</ref> According to the biographer [[Diogenes Laërtius]], Epimenides was a shepherd on the Greek island of [[Crete]].<ref name="Hansen" /><ref name="Rothschild">{{cite book|vauthors = Rothschild CK |title=Paul in Athens: The Popular Religious Context of Acts 17|date=2014|publisher=Mohr Siebeck|location=Tübingen|isbn=978-3-16-153260-3|pages=40–42|url=https://books.google.com/books?id=1zTsvXKcl50C&q=long+sleep+Epimenides+Rip+Van+Winkle+Seven+Sleepers&pg=PA41}}</ref> One day, one of his sheep went missing and he went out to look for it, but became tired and fell asleep in a cave under [[Mount Ida (Crete)|Mount Ida]].<ref name="Hansen" /><ref name="Rothschild" /> When he awoke, he continued searching for the sheep, but could not find it,<ref name="Hansen" /><ref name="Rothschild" /> so he returned to his old farm, only to discover that it was now under new ownership.<ref name="Hansen" /><ref name="Rothschild" /> He went to his hometown, but discovered that nobody there knew him.<ref name="Hansen" /> Finally, he met his younger brother, who was now an old man,<ref name="Hansen" /><ref name="Rothschild" /> and learned that he had been asleep in the cave for fifty-seven years.<ref name="Hansen" /><ref name="Rothschild" />

A far more famous instance of a "long sleep" today is the Christian [[Legendary material in Christian hagiography|legend]] of the [[Seven Sleepers|Seven Sleepers of Ephesus]],<ref name="Hansen"/> in which seven Christians flee into a cave during pagan times in order to escape [[Persecution of Christians in the Roman Empire|persecution]],<ref name="Hansen"/> but fall asleep and wake up 360 years later to discover, to their astonishment, that the Roman Empire is now predominantly Christian.<ref name="Hansen"/> The American author [[Washington Irving]]'s short story "[[Rip Van Winkle]]", first published in 1819 in his collection of short stories ''[[The Sketch Book of Geoffrey Crayon, Gent.]]'',<ref name="Burstein"/><ref>{{cite book| vauthors = Jones BJ |title=Washington Irving: An American Original|location= New York|publisher= Arcade Books|date=2008|pages= 177–178|isbn=978-1-55970-836-4}}</ref> is about a man in [[Colonial history of the United States|colonial America]] named Rip Van Winkle who falls asleep on one of the [[Catskill Mountains]] and wakes up twenty years later after the [[American Revolution]].<ref name="Burstein"/> The story is now considered one of the greatest classics of [[American literature]].<ref name="Burstein"/>

===In studies on consciousness and philosophy===
As an [[altered state of consciousness]], dreamless deep sleep has been used as a way to investigate [[animal consciousness|animal]]/human [[consciousness]] and [[qualia]]. Insights about differences of the living sleeping brain to its wakeful state and the transition period may have implications for potential explanations of human subjective experience, the so-called [[hard problem of consciousness]], often delegated to the realm of [[philosophy]], including [[neurophilosophy]]<ref>{{cite book |last1=Churchland |first1=Patricia Smith |title=A neurophilosophical slant on consciousness research |series=Progress in Brain Research |date=1 January 2005 |volume=149 |pages=285–293 |publisher=Elsevier |language=en}}</ref><ref>{{cite journal |last1=Doerig |first1=Adrien |last2=Schurger |first2=Aaron |last3=Hess |first3=Kathryn |last4=Herzog |first4=Michael H. |title=The unfolding argument: Why IIT and other causal structure theories cannot explain consciousness |journal=Consciousness and Cognition |date=1 July 2019 |volume=72 |pages=49–59 |doi=10.1016/j.concog.2019.04.002 |pmid=31078047 |s2cid=147704603 |language=en |issn=1053-8100|doi-access=free }}</ref><ref>{{cite journal |last1=Tsytsarev |first1=Vassiliy |title=Methodological aspects of studying the mechanisms of consciousness |journal=Behavioural Brain Research |date=February 2022 |volume=419 |article-number=113684 |doi=10.1016/j.bbr.2021.113684|pmid=34838578 |s2cid=244570791 }}</ref><ref>{{cite journal |last1=Kerskens |first1=Christian Matthias |last2=López Pérez |first2=David |title=Experimental indications of non-classical brain functions |journal=Journal of Physics Communications |date=1 October 2022 |volume=6 |issue=10 |page=105001 |doi=10.1088/2399-6528/ac94be |bibcode=2022JPhCo...6j5001K |language=en |issn=2399-6528|doi-access=free|arxiv=1806.07998 }}</ref> (or in some cases to religion and similar approaches).

===In art===
Of the thematic representations of sleep in art, physician and sleep researcher Meir Kryger wrote, "[Artists] have intense fascination with mythology, dreams, religious themes, the parallel between sleep and death, reward, abandonment of conscious control, healing, a depiction of innocence and serenity, and the erotic."<ref name="huffpostart">{{cite news| vauthors = Frank P |title=Why Have Artists Always Found Sleep Such A Fascinating Subject?|url=https://www.huffingtonpost.com/entry/artists-and-sleep-meir-kryger_us_576b1305e4b09926ce5db903|access-date=14 July 2017|work=[[HuffPost]]|date=24 June 2016|archive-url=https://web.archive.org/web/20170725165747/https://www.huffingtonpost.com/entry/artists-and-sleep-meir-kryger_us_576b1305e4b09926ce5db903|archive-date=25 July 2017|url-status=live}}</ref>
<gallery mode="packed" heights="200">
File:Carel Fabritius - De poort bewaker (1654).jpg|''[[The Sentry (painting)|The Sentry]]'' (1654) by [[Carel Fabritius]]
File:Museo del Prado - Goya - Caprichos - No. 43 - El sueño de la razon produce monstruos.jpg|''[[The Sleep of Reason Produces Monsters]]'' (1799) by [[Francisco Goya]]
File:Honoré Daumier - The Second Class Carriage - Walters 371224.jpg|''The Second Class Carriage'' (1864) by [[Honoré Daumier]]
File:Waterhouse-sleep and his half-brother death-1874.jpg|''[[Sleep and his Half-brother Death]]'' (1874) by [[John William Waterhouse]]
File:Илья Е. Репин - Отдых. Портрет В.А.Репинa (1882).jpg|''Taking a Rest'' (1882) by [[Ilya Repin]]
File:George Hare - Victory of Faith.jpg|''[[The Victory of Faith (painting)|The Victory of Faith]]'' (1891) by [[Saint George Hare]]
File:Auf dem Ofen 1895.jpg|''Zwei schlafende Mädchen auf der Ofenbank'' (1895) by [[Albert Anker]]
File:Flaming June, by Frederic Lord Leighton (1830-1896).jpg|''[[Flaming June]]'' ({{circa}} 1895) by [[Frederic Leighton]]
File:Noon, rest from work - Van Gogh.jpeg|''Noon – Rest from Work'' (1890) by [[Vincent van Gogh]] (after [[Jean-François Millet|Millet]])
File:Albert Anker - Schlafendes Mädchen auf einer Holzbank.jpg|''Sleeping Girl on a Wooden Bench'' by [[Albert Anker]]
</gallery>

== See also ==
{{Portal|Biology|Society|Psychology}}
<!-- Please keep entries in alphabetical order & add a short description [[WP:SEEALSO]] -->
{{div col|colwidth=15em|small=no}}
* [[Co-sleeping]]
* [[Hypnogram]]
* [[Microsleep]]
* [[Morvan's syndrome]]
* [[Oleamide]]
* [[Power nap]]
* [[Rheum]]
* [[Sleep epidemiology]]
* [[Sleep in space]]
* [[Sleep-learning]]
* [[Sleep medicine]]
* [[Sleep paralysis]]
* [[Sleeping positions]]
* [[Somnology]]
* [[Somnophilia]]
* [[Start school later movement]]
* [[Sudden infant death syndrome]]
* [[Sudden arrhythmic death syndrome]]
* [[Unconsciousness]]
* [[Yawn]]
* [[Yoga-nidra]]{{div col end}}
<!-- please keep entries in alphabetical order -->

== References ==
{{reflist}}

== Further reading ==
* {{cite journal | vauthors = Brown RE, Basheer R, McKenna JT, Strecker RE, McCarley RW | title = Control of sleep and wakefulness | journal = [[Physiological Reviews]] | volume = 92 | issue = 3 | pages = 1087–187 | date = July 2012 | pmid = 22811426 | pmc = 3621793 | doi = 10.1152/physrev.00032.2011 | ref = Brown }}

== External links ==
{{Commons category|Sleeping|Sleep}}
{{Wikiquote}}
{{Wiktionary}}
* [https://www.nytimes.com/2012/09/23/opinion/sunday/rethinking-sleep.html?pagewanted=1&_r=0&ref=general&src=me Rethinking Sleep], David K. Randall, ''New York Times,'' September 2012
* [https://www.theatlantic.com/magazine/archive/2017/01/how-to-sleep/508781/ How to Sleep], James Hamblin, ''The Atlantic,'' January 2017

{{Neuroscience}}
{{Psychophysiology}}
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[[Category:Sleep| ]]
[[Category:Behavior]]
[[Category:Human behavior]]
[[Category:Mental states]]
[[Category:Unsolved problems in neuroscience]]