Animals sleep for very different amounts of time. Koalas rest or sleep for about 20 hours a day. Their food, eucalyptus leaves, gives them very little energy, so they move very little.

Wild African elephants sleep for only about two hours a day, and they usually sleep standing up. Some birds can even sleep while they are flying! How long an animal sleeps depends on how it lives.

Sleep time varies surprisingly widely among animals. Koalas are said to sleep or rest for about 18 to 22 hours a day, mainly because eucalyptus leaves provide little energy and are hard to digest. In contrast, a 2017 study of two wild African elephants in Botswana found that they slept only about two hours a day, mostly standing up, and lay down only every three or four days.

Birds have even stranger habits. Frigatebirds can sleep while flying, sometimes with only half of the brain asleep. Chinstrap penguins raising chicks take more than 10,000 tiny naps a day, each lasting only a few seconds. Scientists think that body size, diet, predators and safe sleeping places are all linked to how long animals sleep.

Sleep duration differs enormously across the animal kingdom. Koalas are commonly said to sleep or rest for 18 to 22 hours a day, a lifestyle that conserves energy on a diet of eucalyptus leaves, which are low in nutrients and hard to digest. At the other extreme, researchers fitted activity monitors into the trunks of two wild African elephant matriarchs in Botswana. Treating periods when the trunk was completely still as sleep, they estimated about two hours of sleep per day, mostly while standing, and the elephants sometimes went up to 46 hours without sleeping while traveling long distances.

Birds show further variety: brain recordings revealed that frigatebirds sleep in flight, often with one hemisphere at a time, and nesting chinstrap penguins accumulate around 11 hours of sleep through more than 10,000 microsleeps lasting only seconds. Such comparisons, however, require caution. The elephant estimate is based on movement rather than brain activity and involves just two animals, and animals in captivity often sleep longer than wild ones; sloths sleep about 16 hours in zoos but about 10 in the wild. Researchers link these differences to body size, diet, predation and sleeping-site safety, though mostly through correlation.