Which animals sleep the most?

Sleep duration across the animal kingdom varies enormously and follows a fairly logical pattern. At the long end, koalas sleep somewhere around 18 to 22 hours a day, and various bats, opossums, and armadillos are close behind. Domestic cats run 12 to 16 hours, dogs 10 to 14. At the short end, large grazing animals sleep remarkably little: giraffes get roughly four to five hours, often in short bouts, and elephants in the wild have been recorded sleeping as little as two hours a day, sometimes standing.

The drivers are diet and vulnerability. Eucalyptus leaves are low in nutrients and require heavy digestive processing, so koalas conserve energy by sleeping through most of the day, and predators with calorie-dense meals can afford long rest after a kill. Large herbivores must spend most of their waking hours eating to sustain their body mass, and lying down makes them vulnerable, so they sleep briefly and often standing. Some animals do stranger things: dolphins and some birds sleep one brain hemisphere at a time, keeping one eye open and continuing to swim or fly, and migrating birds have been recorded sleeping in flight. Sleep appears to be essentially universal across the animal kingdom, present even in jellyfish and fruit flies, which is one of the stronger arguments that it performs some fundamental biological function rather than being an optional behavior.