Polar night
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.
There are multiple ways to define twilight, the gradual transition to and from darkness when the Sun is below the horizon.
[8] 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.
However, during dense cloud cover, places like the coast of Finnmark (about 70°) in Norway will experience a "day" that is darker than usual.
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.
During nautical polar twilight, the human eye may distinguish general outlines of ground objects at midday but cannot participate in detailed outdoor operations.
The Norwegian town of Longyearbyen, Svalbard, experiences nautical polar twilight from about 11 November until 30 January.
[12] 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.
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.
[15][16] 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.
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.
[18] A similar study was conducted among men who overwintered at Belgrano II, an Argentine research station in Antarctica.
