The animal that sleeps the longest is the koala, which can sleep up to 20-22 hours a day. Koalas are native to Australia and are arboreal, meaning they live in trees. They have a specialized diet of eucalyptus leaves, which are low in nutrition, so they conserve their energy by sleeping for long periods of time. Koalas are also nocturnal animals, which means they are most active at night and sleep during the day. Despite their long hours of sleep, koalas have a relatively small brain and are not considered to be very intelligent animals.