[[Image:European hedgehog (Erinaceus europaeus).jpg|thumb|[[Hedgehog]]s are mostly nocturnal]]
'''NoctlityNocturnality''' is an [[animal behavior]] characterized by being active 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 [[crispscrypsis]], 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 [[desert]]s, where many animals' nocturnal behavior prevents them from losing precious water during the hot, dry daytime. This is an [[adaptation]] that enhances [[osmoregulation]].<ref>N.A.Campbell (1996) ''Biology'' (4th edition) Benjamin Cummings NY. ISBN 0-8053-1957-3</ref>
Many species which are otherwise diurnal exhibit some nocturnal behaviour; for example, many [[seabird]]s and [[sea turtle]]s attend breeding sites or colonies nocturnally to reduce the risk of predation (to themselves or their offspring) but are otherwise diurnal.
