Polar night

Polar night is a phenomenon in the northernmost and southernmost regions of Earth where night lasts for more than 24 hours. This occurs only inside the polar circles. The opposite phenomenon, polar day, or midnight sun, occurs when the Sun remains above the horizon for more than 24 hours.

"Night" is understood as the center of the Sun being below a free horizon. Since the atmosphere refracts sunlight, polar day is longer than polar night, and the area that is affected by polar night is somewhat smaller than the area of midnight sun. The polar circle is located at a latitude 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 rotates with respect to its star significantly more frequently than it 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
The length of polar night varies by latitude, from 24 hours just inside the polar circles to 179 days at the poles. As mentioned, a location experiencing polar night does not mean that the location will be in full darkness; in many cases, due to sunlight being refracted over the horizon, a location experiencing polar night may be in one of the various phases of polar twilight. As in locations experiencing daytime, the middle of the day will typically be the brightest time in locations experiencing polar twilight.

For example, a typical day during polar night in Vadsø, Norway will begin with astronomical 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 astronomical night to end the day).

Types of polar night
As there are various kinds of twilight, there also exist various kinds of 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.

Polar twilight
Polar twilight occurs in areas that are located at the inner border of the polar circles, where the Sun will be on or below the horizon all day on the winter solstice. There is then no true daylight at the solar culmination, only civil twilight. This means that the Sun is below the horizon, but by less than 6°. During civil twilight, there may still be enough light for most normal outdoor activities because of light scattering by the upper atmosphere and refraction. Street lamps may remain on and a person looking at a window from within a brightly lit room may see their reflection even at noon, as the level of outdoor illuminance will be below that of many illuminated indoor spaces. It occurs at latitudes between about 67°24' and 72°34' North or South, when the Sun does not rise, only civil twilight visible.

Northern Hemisphere:


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

Southern Hemisphere:


 * 68° South: June 7 to July 3
 * 69° South: May 30 to July 11
 * 70° South: May 24 to July 18
 * 71° South: May 19 to July 23
 * 72° South: 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 lux) which are not present in any stage of twilight; thus, the midday twilights experienced anywhere inside the polar circles are still "polar night" for this purpose.

Civil polar night
The civil polar night period produces only a faint glow of light visible at midday. It happens when there is no civil twilight and only nautical twilight occurs at the solar culmination. Civil twilight happens when the Sun is between 0 and 6° below the horizon, and civil night when it is lower than that. Therefore, the civil polar night is limited to latitudes above roughly 72° 34', which is exactly 6° inside the polar circle. Nowhere on mainland Europe is this definition met. On the Norwegian territory of Svalbard, however, civil polar night lasts from about 11 November until 30 January. Dikson, in Russia, experiences civil polar night from December 6 to January 6. During dense cloud cover places like the coast of Finnmark (about 70°) in Norway will get a darker "day". On the Canadian territory of Pond Inlet, Nunavut, however, civil polar night lasts from about 16 December until 26 December.

Nautical polar night
During the nautical polar night period, there is no trace of daylight, except around midday. It happens when there is no nautical twilight and only astronomical twilight occurs at the solar culmination. Nautical twilight happens when the Sun is between six and twelve degrees below the horizon. There is a location at the horizon around midday with more light than others because of refraction. During nautical night, the Sun is lower than 12° below the horizon, so nautical polar night is limited to latitudes above roughly 78° 34', which is exactly 12° within the polar circle, or approximately 11.5° from the pole.

The Norwegian territory of Svalbard Ny-Ålesund experiences this from December 12 to 30. Its antipode (-78.91667°N, -168.06667°W) experiences this from June 12 to July 1.

The Canadian territory of Eureka, Nunavut, experiences this from December 2 to January 8. Its antipode (-79.98333°N, 94.06667°W) experiences this from June 1 to July 11.

The Russian territory of Franz Josef Land experiences this from November 27 to January 15. Its antipode (-81°N, -125°W) experiences this from May 25 to July 17.

Alert, Nunavut, the northernmost settlement in Canada and the world, experiences this from November 19 to January 22. Its antipode (-82.5°N, 117.63333°W) experiences this from May 19 to July 25.

Oodaaq, a gravel bank at the northern tip of Greenland and a disputed northernmost point of land, experiences this from November 15 to January 27. Its antipode (-83.66667°N, 150.11667°W) experiences this from May 13 to July 31.

Astronomical polar night
The astronomical polar night is a period of continuous night where no astronomical twilight occurs. Astronomical twilight happens when the Sun is between twelve and eighteen degrees below the horizon and astronomical night when it is lower than that. Thus, the astronomical polar night is limited to latitudes above roughly 84° 34', which is exactly 18° within the polar circle, or approximately five and a half degrees from the pole. The only permanent settlement on Earth above this latitude is Amundsen–Scott South Pole Station, the South Pole scientific research station, whose winter personnel are completely isolated from mid-February to late October. During the astronomical polar night, stars of the sixth magnitude, which are the dimmest stars visible to the naked eye, will be visible throughout the entire day. This happens when the sun is between exactly 18° and approximately 23° 26' below the horizon. These conditions last about 11 weeks at the poles.

South Pole, Antarctica experiences this from May 11 to August 1.

The North Pole experiences this from November 12 to January 28.

Polar Sun cycle
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.

Effects on sleep and mental health
Numerous analyses have been conducted to examine the effects of polar night on humans. In 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 artificial light in Tromsø.

A similar study was conducted among men who overwintered at 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.

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 disorder (3 diagnosed), depressive disorder (1 diagnosed), adjustment disorder (2 diagnosed), and alcohol use disorder (1 diagnosed).

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. 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; therefore, the polar night was shown to have sleep and/or mental health effects in all three studies.