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Polar night is a phenomenon that occurs in the northernmost and southernmost regions of Earth when the Sun remains below the horizon for more than 24 hours. This only occurs inside the . The opposite phenomenon, polar day or midnight sun, occurs when the Sun remains above the horizon for more than 24 hours.
There are multiple ways to define twilight, the gradual transition to and from darkness when the Sun is below the horizon. "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. "Nautical" twilight continues until the Sun is 12 degrees below the horizon. During nautical twilight, the horizon is visible enough for navigation. "Astronomical" twilight continues until the Sun has sunk 18 degrees below the horizon. Beyond 18 degrees, refracted sunlight is no longer visible. True night is defined as the period when the sun is 18 or more degrees below either horizon.
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 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 with respect to its star significantly more frequently than it Orbital period the star (and with no tidal locking between the two) will experience the same phenomenon (a nighttime lasting more than one rotation period).
For example, a typical day during civil polar twilight in 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).
Northern Hemisphere:
Southern Hemisphere:
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 nights" for this purpose.
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, 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.
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 () experiences this from about June 12 to July 1. The Canada research base of Eureka, Nunavut, experiences this from about December 2 to January 8. Its antipode () experiences this from about June 1 to July 11. The Russian territory of Franz Josef Land experiences this from about November 27 to January 15. Its antipode () experiences this from about May 25 to July 17. Alert, Nunavut, the northernmost settlement in Canada and the world, experiences this from about November 19 to January 22. Its antipode () experiences this from about 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 about November 15 to January 27. Its antipode () experiences this from about May 13 to July 31.
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° 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 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, while the North Pole experiences this from about November 12 to January 28.
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 (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.
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