Do animals sleep? Yes, but you cannot always tell by looking. A sleeping ostrich sits still with its eyes open and its neck upright, so it looks alert. Yet recordings show that in its deepest sleep, its brain waves are slow and strong. So how do scientists know when an animal is really asleep? Scientists use more than one test.

One way is to read the brain. To record ostrich sleep, scientists put helmets with electrodes on six ostriches. The helmets measured the birds' body temperature, brain signals, eye movements and neck muscles. The readings clearly showed when the birds were asleep. That is one way to be sure that an animal is asleep.

Another way is to see how an animal reacts. Sperm whales float upright, almost still, with their heads at or just below the surface. Were they really asleep? Researchers brought a boat near floating whales, and the whales did not react, even though both eyes could see the boat, until it accidentally touched one of them.

You may have heard that dolphins never sleep. In fact, a dolphin sleeps with one half of its brain resting while the other half stays awake. Some birds sleep with half a brain, too. A bird that does this closes only the eye on the sleeping side, and the other eye stays open.

An octopus has two kinds of sleep. In quiet sleep, it is pale and still, and its pupils narrow to slits. In active sleep, its skin flashes changing colors and textures, and its eyes, arms and suckers move.

Where scientists measure sleep can change the answer. To measure sleep in wild sloths, they fitted a small, light cap of brain sensors, plus a radio collar. In the rainforest, wild three-toed sloths slept only about 9.6 hours a day. Older studies of sloths in captivity had said about 16 hours. Captive sloths may have been more relaxed, with no predators and food always supplied. Measuring brain signals only in captivity can lead to the wrong conclusions about wild animals.

How much animals sleep varies a lot. A koala sleeps or rests for 18 to 22 hours a day, because eucalyptus leaves are very low in energy and slow to digest. An elephant sleeps only about 2 to 3 hours a day. An elephant seal sleeps 10 to 11 hours on land but just 2 hours a day at sea. On deep dives of about 30 minutes, it naps for about 10 minutes at a time while it holds its breath.

Can a bird sleep while flying? Alpine swifts may hardly ever land. Scientists tagged six of them, and the data showed the birds stayed in the air for about 200 days. Great frigatebirds were tracked on flights of up to ten days and 3000 kilometers. If these birds sleep on those trips, they must do it in the air.

Not every deep rest is sleep. Hibernation is not just a long sleep. It is better understood as a state called torpor. A black bear in winter sleep lets its body temperature fall only about five degrees Celsius. Hummingbirds can go into torpor at night, too. As of November 2026, scientists report that on a very cold night a hummingbird's metabolism, its body's energy use, can drop by as much as 95 percent.

So do animals sleep? Yes, from big birds to tiny fruit flies. Fruit flies sleep. Upside-down jellyfish are less active at night, pulsing less. A sleeping fish slows down, and its gills pump less. Animals sleep in many shapes and sizes, and some of them do it in ways you would never guess.

Words to know

• electrodes: sensor pads
• upright: straight up
• torpor: deep rest
• metabolism: body's energy use