Nocturnality is an animal behavior characterized by activity during the night and sleeping during the day. The opposite is diurnality. The intermediate crepuscular schedule (twilight activity) is also common. Some species are active both in daytime and at night. Living at night can be seen as a form of niche differentiation, where a species' niche is partitioned not by resources but by time itself, i.e. temporal division of the ecological niche. It can also be viewed as a form of crypsis, in other words an adaptation to avoid or enhance predation. There are other reasons for nocturnality as well, such as keeping out of the heat of the day. This is especially true in deserts, where many animals' nocturnal behavior prevents them from losing precious water during the hot, dry daytime. This is an adaptation that enhances osmoregulation.[1]

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 diurnal.

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 (see metaturnal). Others, e.g. bushbabies and bats, can only function at night.

A person who is nocturnal is referred to as a night owl; he or she is of the "eveningness" chronotype.

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