The midnight sun is a phenomenon, occurring in latitudes north of the arctic circle and south of the antarctic circle, where the sun is visible during at least 24 hours. Since there are no permanent human settlements south of the antarctic circle, the countries and territories whose population experiences it are limited to Alaska, northern Canada, Greenland, Iceland (due to refraction), northern Norway, Sweden and Finland, and extremities of Russia such as Novaya Zemlya or Murmansk.
The opposite phenomenon, polar night, is observed in winter when the sun is sufficiently below the horizon, so that there is insufficient sunlight or no light at all. (A lingering dusk may occur, if the sun is only 12 degrees or less below the horizon.)
Since the earth's axis is tilted with respect to the ecliptic by approximately 23 degrees 26 minutes (commonly rounded to 23 degrees and a half), the sun does not set at high latitudes in (local) summer. The duration of the midnight sun increases from one day during the summer solstice at the polar circle to approximately six months at the poles. At extreme latitudes, it is usually referred to as polar day. The length of the time when the sun is above the horizon varies from 20 hours at the Arctic Circle and Antarctic Circle to 186 days at the Poles.
Due to refraction, the midnight sun may be experienced at latitudes slightly below the polar circle, though not exceeding a degree (depending on local conditions). For example, it is possible to experience the midnight sun in Iceland, even though it is slightly south of the arctic circle.
Locations at latitudes up to 60 degrees experience midnight twilight instead. The sun is just below the horizon, so that daytime activities, such as reading, are still possible without resorting to artificial light. These are the white nights experienced, for example, in St. Petersburg, Russia from about 11 June to 11 July.
Influence on humans
The period of midnight sun is reportedly very taxing on the human body. Peaks in the suicide figures and increased severity of mental disorders have been demonstrated to occur in summer months. Polar nights affect people more severely. People who suffer from seasonal affective disorder are particularly susceptible.
- Explanation of various related phenomona (http://collections.ic.gc.ca/simply_science/toc.htm)