A
nocturnal animal is one that sleeps during the
day and is active at
night - the opposite of the normal (
diurnal) schedule. 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, eg Bushbabies[?] and bats, can only function at night.