Nocturnal

adj. of the night; functioning or active during the night

As an animal behavior, nocturnality describes sleeping during the daytime and being active at night - the opposite of the diurnal human lifestyle, and that of those animals with which we are most familiar. The intermediate crepuscular schedule (twilight activity) is also common. Some species are active both during the day and 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.
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Adjective
1. belonging to or active during the night; "nocturnal animals are active at night"; "nocturnal plants have flowers that open at night and close by day"
(antonym) diurnal
(see-also) nighttime
(classification) biology, biological science
2. of or relating to or occurring in the night; "nocturnal darkness"
(pertainym) night, nighttime, dark
3. of or during or relating to the night; "a nocturnal journey"; "nocturnal stillness"; "nocturnal predators"
(similar) nighttime

(n.)
An instrument formerly used for taking the altitude of the stars, etc., at sea.
  

(a.)
Of, pertaining to, done or occuring in, the night; as, nocturnal darkness, cries, expedition, etc.; -- opposed to diurnal.
  

(a.)
Having a habit of seeking food or moving about at night; as, nocturnal birds and insects.
  

- Related to nighttime, or occurring at night.