View source for Polar night

{{Short description|Phenomenon where the Sun remains below the horizon for more than 24 hours}}
{{More citations needed|date=October 2022}}
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| caption1          = Polar night and [[Aurora|aurora australis]] at the [[South Pole]], [[Antarctica]].
| image2            = Polar-Night Longyearbyen.jpg
| caption2          = Characteristic nautical ([[Blue hour|blue]]) polar twilight in [[Longyearbyen]], [[Svalbard]], [[Norway]].
| image3            = Polar night in Naryan-Mar.jpg
| caption3          = Civil polar twilight in [[Naryan-Mar]], [[Russia]].
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'''Polar night''' is a phenomenon that occurs in the [[polar regions of Earth|northernmost and southernmost regions of Earth]] when the [[Sun]] remains below the [[horizon]] for more than 24&nbsp;hours. This only occurs inside the [[polar circle]]s.<ref>{{Cite book |last=Burn |first=Chris |url=http://nwtresearch.com/sites/default/files/the-polar-night.pdf |title=The Polar Night |publisher=The Aurora Research Institute |access-date=28 September 2015}}</ref> The opposite phenomenon, polar day or [[midnight sun]], occurs when the Sun remains above the horizon for more than 24&nbsp;hours. 

There are multiple ways to define twilight, the gradual transition to and from darkness when the Sun is below the horizon.{{sfn|Greene|2003|p=86}} "Civil" twilight occurs when the Sun is between 0 and 6 degrees below the horizon. Nearby planets like [[Venus]] and bright stars like [[Sirius]] are visible during this period.{{sfn|Mason|1933|p=690}} "Nautical" twilight continues until the Sun is 12 degrees below the horizon.{{sfn|Ottewell|2019}} During nautical twilight,  the horizon is visible enough for navigation.{{sfn|Kher|Bikos|n.d.}} "Astronomical" twilight continues until the Sun has sunk 18 degrees below the horizon.{{sfn|Mason|1933|p=690}}{{sfn|Shubinski|2023|loc="In twilight"}} Beyond 18 degrees, refracted sunlight is no longer visible.{{sfn|Shubinski|2023|loc="In twilight"}} True night is defined as the period when the sun is 18 or more degrees below either horizon.{{sfn|Ottewell|2019}}

Since the atmosphere refracts sunlight, polar day is longer than polar night, and the area that experiences polar night is slightly smaller than the area that experiences polar day. The polar circles are located at [[latitude]]s between these two areas, at approximately 66.5°. While it is day in the [[Arctic Circle]], it is night in the [[Antarctic Circle]], and vice versa.

Any planet or moon with a sufficient [[axial tilt]] that [[Rotation period|rotates]] with respect to its star significantly more frequently than it [[Orbital period|orbits]] the star (and with no [[tidal locking]] between the two) will experience the same phenomenon (a nighttime lasting more than one rotation period).

==Description==
{{Unreferenced section|date=October 2022}}
[[File:Ruka Cottage (8328890447).jpg|thumb|right|Civil polar twilight in [[Ruka, Finland]] at noon in December]]
The length of polar night varies by latitude from 24 hours just inside the polar circles to 179 days at the poles. As there are various kinds of [[twilight]], there also exist various kinds of polar twilight that progress towards true polar night. Each kind of polar night is defined as when it is darker than the corresponding kind of twilight. The descriptions below are based on relatively clear skies, so the sky will be darker in the presence of dense clouds.

==Types of polar night==
{{More citations needed section|date=October 2022}}
[[File:Morketidettermiddag.JPG|thumb|right|Early afternoon during civil polar twilight in [[Tromsø (city)|Tromsø]], [[Norway]].]]
[[File:Nordkinnhalvoya-polar-night.jpg|thumb|right|Civil polar twilight on [[Nordkinn Peninsula]] in Norway, mainland Europe's northernmost peninsula.]]

===Polar twilight===
As mentioned, a location experiencing polar night does not mean that the location will be in full darkness; in most cases, due to sunlight being [[Refraction|refracted]] over the horizon, a location experiencing polar night will actually be in one of the various phases of polar twilight. As in locations experiencing daylight, the middle of the day will typically be the brightest time in locations experiencing polar twilight.<ref>{{Cite web |last=Mulvaney |first=Kieran |date=2024-02-01 |title=What is polar night? |url=https://www.nationalgeographic.com/environment/article/polar-night-arctic-antarctic |access-date=2024-02-28 |website=Environment |language=en}}</ref>

For example, a typical day during civil polar twilight in [[Vadsø Municipality|Vadsø, Norway]] will begin with night, astronomical twilight, nautical twilight, and civil twilight in that order (with each successive phase including more light than the last). Following civil twilight, the day will progress through the other phases in the opposite order (nautical twilight, then astronomical twilight, then night to end the day).<ref>{{Cite web |date=2023-01-12 |title=The Polar Night and the Polar Day - Aurora Labs Norway |url=https://auroralabsnorway.com/the-polar-night-and-the-polar-day/ |access-date=2024-03-07 |language=en-US}}</ref> 

====Civil polar twilight====
Civil polar twilight occurs at latitudes above 72°34' North or South, where the Sun will be below the horizon all day on the [[winter solstice]], but by less than 6° at [[solar noon]].<ref>{{cite web |url=https://basecampexplorer.com/spitsbergen/arctic-dictionary/polar-night/   |title=Polar Night|author=<!--Not stated--> |date= |website=Base Camp Explorer |publisher= |access-date=February 6, 2025}}</ref> There is then no true [[daylight]] at the [[solar culmination]], only [[civil twilight]]. During civil polar twilight, there is still enough light for most normal outdoor activities at midday because of light scattering by the upper atmosphere and refraction. However, during dense cloud cover, places like the coast of [[Finnmark]] (about&nbsp;70°) in [[Norway]] will experience a "day" that is darker than usual. [[Street lamp]]s may therefore remain on even at midday, and a person looking at a window from within a brightly lit room might still be able to see their reflection, as the level of outdoor [[illuminance]] will be below that of many illuminated indoor spaces. 

Northern Hemisphere:

*68° North: about December 9 to January 2
*69° North: about December 1 to January 10
*70° North: about November 26 to January 16
*71° North: about November 21 to January 21
*72° North: about November 16 to January 25

Southern Hemisphere:

*68° South: about June 7 to July 3
*69° South: about May 30 to July 11
*70° South: about May 24 to July 18
*71° South: about May 19 to July 23
*72° South: about May 14 to July 27

Sufferers of [[seasonal affective disorder]] tend to seek out therapy with artificial light, as the psychological benefits of daylight require relatively high levels of ambient light (up to 10,000&nbsp;[[lux]]) which are not present in any stage of twilight; thus, the midday twilights experienced anywhere inside the polar circles are still "polar nights" for this purpose.

====Nautical polar twilight====
Nautical polar twilight occurs at latitudes between about 72°&nbsp;34' and 78°&nbsp;34' North or South, which is exactly 6° to 12° inside the polar circles. There is then no [[civil twilight]] at the [[solar culmination]], only [[nautical twilight]]. During nautical polar twilight, the human eye may distinguish general outlines of ground objects at midday but cannot participate in detailed outdoor operations.<ref>{{Cite web |url=http://aa.usno.navy.mil/faq/docs/RST_defs.php |title=Rise, Set, and Twilight Definitions |access-date=2007-09-06 |archive-date=2015-08-14 |archive-url=https://web.archive.org/web/20150814180458/http://aa.usno.navy.mil/faq/docs/RST_defs.php |url-status=dead }}</ref> 

Civil twilight happens when the Sun is between 0 and 6° below the horizon, so this phenomenon can also be referred to as civil polar night. Nowhere on mainland Europe is this definition met. The Norwegian town of [[Longyearbyen]], [[Svalbard]], experiences nautical polar twilight from about 11 November until 30 January. [[Dikson (urban-type settlement)|Dikson]], in Russia, experiences nautical polar twilight from about 6 December to 6 January. On the Canadian territory of [[Pond Inlet, Nunavut]], nautical polar twilight lasts from about 16 December until 26 December.

====Astronomical polar twilight====
Astronomical polar twilight occurs at latitudes between about 78°&nbsp;34' and 84°&nbsp;34' North or South, which is exactly 12° to 18° inside the polar circles. There is then no [[nautical twilight]] at the [[solar culmination]], only [[astronomical twilight]]. During astronomical polar twilight, the sky is dark enough at midday to permit astronomical observation of point sources of light such as [[star]]s, except in regions with more intense [[skyglow]] due to [[light pollution]], [[moonlight]], [[aurora]]s, and other sources of light. There is a location at the [[horizon]] with more light than others around midday due to refraction. Some critical observations, such as of faint diffuse items such as [[nebula]]e and [[galaxy|galaxies]], may require observations beyond the limit of astronomical twilight. 

Nautical twilight happens when the Sun is between 6 and 12° degrees below the horizon, so this phenomenon can also be referred to as nautical polar night. The Norwegian town of [[Ny-Ålesund]], [[Svalbard]], experiences this from about December 12 to 30. Its antipode ({{coord|78|55|S|168|4|W}}) experiences this from about June 12 to July 1. The [[Canada|Canadian]] research base of [[Eureka, Nunavut]], experiences this from about December 2 to January 8. Its antipode ({{coord|79|59|S|94|4|E}}) experiences this from about June 1 to July 11.<ref>{{Cite web |title=Time and Date.com - Eureka, Nunavut, Canada |url=https://www.timeanddate.com/sun/canada/eureka |access-date=March 26, 2024 |publisher=Time and Date.com}}</ref> The Russian territory of [[Franz Josef Land]] experiences this from about November 27 to January 15. Its antipode ({{coord|81|S|125|W}}) experiences this from about May 25 to July 17. [[Alert, Nunavut|Alert]], [[Nunavut]], the northernmost settlement in [[Canada]] and the world, experiences this from about November 19 to January 22. Its antipode ({{coord|82|30|S|117|38|E}}) experiences this from about May 19 to July 25.<ref>{{Cite web |title=Time and Date.com - Alert, Nunavut, Canada |url=http://www.timeanddate.com/sun/canada/alert |access-date=May 14, 2019 |publisher=Time and Date.com}}</ref> [[Oodaaq]], a gravel bank at the northern tip of [[Greenland]] and a disputed [[northernmost point of land]], experiences this from about November 15 to January 27. Its antipode ({{coord|83|40|S|150|7|E}}) experiences this from about May 13 to July 31.<ref>{{Cite web |title=Time and Date.com - Oodap Qeqertaa, Greenland |url=https://www.timeanddate.com/sun/@3426180 |access-date=May 16, 2019 |publisher=Time and Date.com}}</ref>

===True polar night===
A true polar night is a period of continuous [[night]] where no [[astronomical twilight]] occurs at the [[solar culmination]]. During a true polar night, stars of the sixth magnitude, which are the dimmest stars visible to the naked eye, will be visible throughout the entire 24-hour [[day]]. At [[solar noon]], the sun will be between exactly 18° and approximately 23° 26' below the horizon. These conditions last for about 11 weeks at the poles.

Astronomical twilight happens when the Sun is between 12 and 18° degrees below the horizon, so this phenomenon can also be referred to as astronomical polar night. True polar night is limited to latitudes above roughly 84°&nbsp;34' North or South, which is exactly 18° within the polar circles, or approximately five and a half degrees from the poles. The only permanent settlement on Earth at these latitudes is the [[Amundsen–Scott South Pole Station|Amundsen–Scott scientific research station]] in [[Antarctica]], whose winter personnel are completely isolated from mid-February to late October. The [[South Pole]] experiences this from about May 11 to August 1,<ref>{{Cite web |title=Time and Date.com - South Pole, Antarctica |url=https://www.timeanddate.com/sun/antarctica/south-pole |access-date=June 1, 2019 |publisher=Time and Date.com}}</ref> while the [[North Pole]] experiences this from about November 12 to January 28.<ref name="arctic polar night livescience">{{Cite web |last=Rao |first=Joe |date=21 September 2010 |title=The Myth of Arctic Daylight and Darkness Exposed |url=http://www.livescience.com/32814-arctic-daylight-darkness-myth-equinox.html |access-date=14 April 2017 |website=[[Live Science]]}}</ref><ref>{{Cite web |title=Time and Date.com - North Pole |url=https://www.timeanddate.com/sun/@90.0000000,0.0000000 |access-date=March 12, 2024 |publisher=Time and Date.com}}</ref>

==Polar Sun cycle==
[[File:Optical effect march sunset - NOAA.jpg|thumb|upright=1.3|At [[Earth's poles]] the Sun appears at the horizon only and all day around [[equinox]], marking the change between the half year long polar night and [[polar day]]. The picture shows the [[South Pole]] right before March equinox, with the Sun appearing through [[refraction]] despite being still below the horizon.]]

If an observer located on either the North Pole or the South Pole were to define a "day" as the time from the maximal elevation of the Sun above the horizon during one period of daylight, until the maximal elevation of the Sun above the horizon of the next period of daylight, then a "polar day" as experienced by such an observer would be one Earth-year long.<ref>[https://www-istp.gsfc.nasa.gov/stargaze/StarFAQ10.htm#q164 NASA: The Sun and Seasons] NASA.  (See last paragraph, section 164.)  By David Stern.  Last updated Sept. 17, 2004.  Downloaded Feb. 17, 2017.</ref>

== Effects on sleep and mental health ==
Numerous analyses have been conducted to examine the effects of polar night on humans. In [[Tromsø (city)|Tromsø, Norway]], a city located at 69 degrees north, there is a 2 month long polar night, lasting from mid-November to mid-January. An analysis was conducted based on 2015-16 data from a health survey that involved residents of the region over age 40, with the goal being to analyze the seasonal variation of sleeping patterns in Tromsø. The study found that there was a higher prevalence of [[insomnia]] among men in the fall and winter months, but not among women. However, overall, sleep duration varied little to none throughout the year despite the extreme changes in daylight; it is worthwhile to note that a factor in this result may be the significant amount of [[Lighting|artificial light]] in Tromsø.<ref name=":0">{{Cite journal |last1=Sivertsen |first1=Børge |last2=Friborg |first2=Oddgeir |last3=Pallesen |first3=Ståle |last4=Vedaa |first4=Øystein |last5=Hopstock |first5=Laila Arnesdatter |date=2021-03-04 |title=Sleep in the land of the midnight sun and polar night: The Tromsø study |url=https://www.tandfonline.com/doi/full/10.1080/07420528.2020.1845191 |journal=Chronobiology International |language=en |volume=38 |issue=3 |pages=334–342 |doi=10.1080/07420528.2020.1845191 |issn=0742-0528 |pmid=33249932 |s2cid=227237010 |hdl-access=free |hdl=11250/2755604}}</ref>

A similar study was conducted among men who overwintered at [[Belgrano II Base|Belgrano II]], an Argentine research station in [[Antarctica]]. The station is located at 77 degrees south, resulting in a polar night 4 months in length. The study was conducted across 5 different winter campaigns in the 2010s, bringing in a total of 82 participants. The study found that participants generally slept for longer periods of time in the summer months than the winter months. Additionally, greater amounts of [[social jetlag]] were observed in the winter months.<ref name=":1">{{Cite journal |last1=Tortello |first1=C. |last2=Folgueira |first2=A. |last3=Lopez |first3=J. M. |last4=Didier Garnham |first4=F. |last5=Sala Lozano |first5=E. |last6=Rivero |first6=M. S. |last7=Simonelli |first7=G. |last8=Vigo |first8=D. E. |last9=Plano |first9=S. A. |date=2023-09-24 |title=Chronotype delay and sleep disturbances shaped by the Antarctic polar night |journal=Scientific Reports |language=en |volume=13 |issue=1 |pages=15957 |bibcode=2023NatSR..1315957T |doi=10.1038/s41598-023-43102-0 |issn=2045-2322 |pmc=10518309 |pmid=37743400}}</ref>

A third study aimed to examine the mental health of 88 Korean crew members at two different research stations in Antarctica, [[King Sejong Station]] and [[Jang Bogo Station]]. No crew members had been diagnosed with a [[mental illness]] prior to the study. While in Antarctica, 7 of the 88 crew members were diagnosed with a mental illness during early winter. The mental illnesses included [[Insomnia|insomnia disorder]] (3 diagnosed), [[depressive disorder]] (1 diagnosed), [[adjustment disorder]] (2 diagnosed), and [[alcohol use disorder]] (1 diagnosed).<ref name=":2">{{Cite journal |last1=Kang |first1=Jae Myeong |last2=Cho |first2=Seong-Jin |last3=Cho |first3=Seo-Eun |last4=Bang |first4=Taemo |last5=Chae |first5=Byung Do |last6=Yi |first6=Eojin |last7=Bae |first7=Seung Min |last8=Na |first8=Kyoung-Sae |last9=Jung |first9=Jaehun |last10=Kang |first10=Seung-Gul |date=2022-08-10 |title=Mood and Sleep Status and Mental Disorders During Prolonged Winter-Over Residence in Two Korean Antarctic Stations |journal=Nature and Science of Sleep |language=English |volume=14 |pages=1387–1396 |doi=10.2147/NSS.S370659 |pmc=9379312 |pmid=35982827 |doi-access=free}}</ref>

Overall, both Antarctic studies showed a lower amount of sleep beginning at the start of winter, while the study from the Korean bases also showed an onset of mental health problems at that time.<ref name=":1" /><ref name=":2" /> While the study from Tromsø did not show a similar drop in sleep duration as the Antarctic studies (perhaps due to the high amounts of artificial light), it did show an increased amount of insomnia in men during winter;<ref name=":0" /> therefore, the polar night was shown to have sleep and/or mental health effects in all three studies.

==See also==
*[[Midnight sun]]

==References==
{{reflist}}

==Further reading==
* {{Cite book |last=Greene |first=David |title=Light and Dark |date=2003 |publisher=Institute of Physics Publishing |isbn=0-7503-0874-5 |edition=eBook}}
* {{Cite magazine |last=Mason |first=W. A. |date=May 1933 |title=Twilight |magazine=United States Naval Institute Proceedings |volume=59 |issue=363 |issn=0041-798X}}
* {{Cite web |last=Ottewell |first=Guy |date=14 June 2019 |title=The undark nights of summer |url=https://earthsky.org/earth/the-undark-nights-of-summer-guy-ottewell/ |access-date=21 April 2024 |website=EarthSky}}
* {{Cite web |last1=Kher |first1=Aparna |last2=Bikos |first2=Konstantin |date=n.d. |title=Twilight, Dawn, and Dusk |url=https://www.timeanddate.com/astronomy/different-types-twilight.html |access-date=21 April 2024 |website=Time and Date |language=en}}
* {{Cite web |last=Shubinski |first=Raymond |date=9 November 2023 |title=Discover the three kinds of twilight |url=https://www.astronomy.com/observing/twilights-glow-is-calling/ |access-date=20 April 2024 |website=Astronomy Magazine}}
* {{Cite journal |last1=Hansen |first1=V. |last2=Lund |first2=E. |last3=Smith-Sivertsen |first3=T. |date=March 1998 |title=Self-reported mental distress under the shifting daylight in the high north |journal=[[Psychological Medicine]] |volume=28 |issue=2 |pages=447–452 |doi=10.1017/S0033291797006326 |doi-broken-date=13 April 2025 |pmid=9572101 |s2cid=10556073}}

==External links==
{{Commons category|Polar night}}
* [https://web.archive.org/web/20120904173515/http://collections.ic.gc.ca/simply_science/toc.htm The polar night and polar darkness]
* [http://weather.cs.uit.no/video/index-2006.html Many years' of webcam pictures from Tromsø, Norway.]  These clearly show the progression into and away from winter at a latitude within the Arctic Circle.

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[[Category:Earth phenomena]]
[[Category:Polar regions of the Earth]]
[[Category:Night]]