Nocturnality

A nocturnal animal is one that sleeps during the day and is active at night - the opposite of the typical human schedule (diurnal). The intermediate crepuscular schedule (twilight activity) is also common. Some species are active both during the day and night.

Many species which are otherwise diurnal exhibit some nocturnal behaviour, for example many seabirds and sea turtles attend breeding sites or colonies nocturnally to reduce the risk of predation (to themselves or their offspring) but are otherwise dirurnal.

Nocturnal animals generally have highly developed senses of hearing and smell, and specially adapted eyesight. In zoos, nocturnal animals are usually kept in special night-illumination enclosures to reverse their normal sleep-wake cycle and to keep them active during the hours when visitors will be attempting to see them.

Some animals, such as cats, have eyes that can adapt to both night and day levels of illumination. Others, e.g. bushbabies and bats, can only function at night.

A person who exhibits nocturnal habits is referred to as a night owl. Night owls who have difficulty adopting normal sleeping and waking times may have delayed sleep phase syndrome.

Examples

The following animals are generally classified as nocturnal: