Night

Night is the part of the daily cycle when a particular place on Earth is turned away from the Sun and receives little or no direct sunlight. It generally follows evening and comes before morning, although the exact boundaries of night depend on how the term is defined. After sunset, the sky does not become completely dark immediately.

A period called twilight occurs while sunlight still illuminates the upper atmosphere, and darkness gradually increases as the Sun moves farther below the horizon. During night, stars and planets can become visible because the bright sunlight scattered through the atmosphere is greatly reduced. The length of nighttime changes throughout the year and varies with latitude.

Near the Earth's polar regions, seasonal differences can become extreme, with very long periods of daylight or darkness. Night is also closely connected with the biological rhythms of many living organisms. In humans, the circadian rhythm helps coordinate sleep and wakefulness over a cycle of roughly 24 hours. Darkness is an important environmental signal for this rhythm, and the body normally increases the production of melatonin in the evening and during darkness, helping prepare for sleep.

Artificial light can influence this process, especially when it is strong or used late at night. Many animals are also adapted to nighttime activity: some, such as bats and many owls, are mainly nocturnal, while others rest during the night and are active during the day. Night therefore is not simply a period when everything stops; it is a distinct part of Earth's daily cycle with important effects on organisms and ecosystems.