The midnight sun is a phenomenon, occurring in latitudes above the polar circle, where the sun is visible during all 24 hours of the day. It is mostly observed in the Arctic, because there are no permanent human settlements south of the Antarctic Circle. Thus, the countries and territories whose population experiences it are limited to Alaska, northern Canada, Greenland, Iceland, northern Norway, Sweden and Finland, and extremities of Russia such as Novaya Zemlya.

Since the earth's axis is tilted with respect to the ecliptic by approximately 23 degrees 37 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 six months at the poles. At extreme latitudes, it is usually referred to as polar day.

A corresponding phenomenon in winter is that the sun does not rise for some time, though lingering dusk may occur if it is 12 degrees or less below the horizon.

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).

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.

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. The period in the local winter when there is almost no sunlight also has drastic consequences on people and can trigger depression in many people. People who suffer from seasonal affective disorder are particularly susceptible.