Night, the period of darkness caused by the disappearance of the sun below the horizon. The limits of night are not firmly defined. Night can be considered as the period from sunset to sunrise, or as the period from the end of evening twilight to the beginning of morning twilight. The length of night in most parts of the world changes with the seasons.

For humans, night is a period of natural rest, but many other animals are more active by night than by day. Many insects, fishes, reptiles, birds, and mammals have eyes that are adapted to faint night light.