Polar night
The polar night is the night lasting more than 24 hours, usually inside the polar circles. The opposite phenomenon, when the sun stays above the horizon for a long time is called the polar day, or midnight sun.
A common misunderstanding is that at each point inside the polar circle, or that at each place where midnight sun occurs, the shortest day is totally dark. Because of twilight, this is not the case. In fact, polar regions get more light throughout the year than regions located closer to the equator. For example, during seven months, Kiruna at 67° 49′ N, (slightly north of the Arctic Circle) has more light than Stockholm at 59° 39′ N.
In regions inside the polar circles, the length of the time when the sun is below the horizon varies from 20 hours at the Arctic Circle and Antarctic Circle to 179 days at the Poles. However not all this time is classified as polar night, since there may be plenty of sunlight because of refraction. Also, one might notice that the time above the horizon at the poles is said to be 186 days. The asymmetry in numbers is because the time when the sun is partially above the horizon is counted towards the "daytime".
Kinds of polar night
Various kinds of polar night exist. This is because polar night is the period during which no twilight occurs; but there are various kinds of twilight.
Civil polar night
The civil polar night is the period during which no civil twilight occurs. Civil twilight happens when the sun is between zero and six degrees below the horizon. Because of refraction, there still is enough light for normal outdoor activities. Civil polar night is limited to latitudes above 72° 33′, which is exactly six degrees inside the polar circle. In mainland Europe, there are no places fulfilling this definition.
Nautical polar night
The nautical polar night is the period during which there is only a faint glow of light visible during midday. It happens when there is no nautical twilight. Nautical twilight happens when the sun is between six and twelve degrees below the horizon. Because of refraction, there is still a place at the horizon with clearly more light than other places. The nautical polar night is limited to latitudes above 78° 33′, which is exactly 12 degrees within the polar circle, or eleven and a half degrees from the pole.
Astronomical polar night
Finally, the astronomical polar night is the period that no trace of light can be seen anywhere and no astronomical twilight occurs. Astronomical twilight happens when the sun is between twelve and eighteen degrees below the horizon. Thus, astronomical polar night happens at latitudes above 84° 33′, which is exactly 18 degrees within the polar circle, or five and a half degrees from the pole.
There are no permanent settlements anywhere in this range of latitude. This portion of the Arctic Ocean is generally permanently ice capped. Some scientific stations in Antarctica, including Amundsen-Scott at the South Pole, experience this.
Effects on humans
The period of polar night is reportedly very taxing on the human body and can trigger depression in many people. Polar days affect people as well. People who suffer from seasonal affective disorder are particularly susceptible.
The effect is so significant that the Finnish language has a word kaamos which specifically refers to the polar night.