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