Do animals sleep? The answer depends on what you count as sleep. Scientists call a rest sleep when the animal is quiet, reacts less to things around it, and sleeps more after being kept awake. Appearances can be deceiving. In its deepest kind of sleep, an ostrich sits still with its eyes open and its neck upright, so it seems alert. To discover the truth, scientists put helmets with electrodes on six ostriches. The helmets measured body temperature, brain activity, eye movements and neck muscles, and the readings clearly showed when the birds were asleep.

One part of the test is reacting less, and it can be checked in the wild. Sperm whales float upright, almost motionless, with their heads at or just below the surface of the sea. Researchers brought a boat close to floating whales, and the whales did not react, even though both of their eyes could see it, until the boat accidentally touched one. Octopuses were shown crab pictures and vibrations, and their delayed response, or total lack of reaction, suggested they were asleep.

Sleeping while flying is a harder test. Great frigatebirds were tracked on flights of up to ten days and 3000 kilometers. To measure their sleep, scientists attached a small recorder to the head that registered the activity of both brain halves. The birds could sleep in flight, using one half of the brain at a time or both halves together, but only for about 0.69 hours a day. Common swifts were tracked with 19 lightweight loggers that recorded light, acceleration and body position. They spent about ten months of the year almost constantly in the air, and at most 0.64 percent of migration was spent on the ground.

Some animals sleep one half-brain at a time. Dolphins are known for unihemispheric sleep, in which half the brain sleeps while the other half stays active. A northern fur seal shows a strongly lopsided brain-wave pattern, with one half of its brain more asleep than the other. A fur seal can even briefly open one eye while a recording shows one half of its brain asleep. Birds do it too, and a bird that sleeps with half its brain closes only the eye on the sleeping side.

Elephants sleep very little, about 2 to 3 hours a day. Two wild African elephant matriarchs fitted with trackers slept about 2 hours a day on average, mostly between 2 a.m. and 6 a.m. One matriarch went without sleep for up to 46 hours and walked about 30 km in 10 hours.

Elephant seals sleep in the sea, and scientists recorded their brain activity with a watertight cap. At sea the seals averaged just 2 hours of sleep a day, compared with 10 to 11 hours on land. On deep dives of about 30 minutes, an elephant seal naps for about 10 minutes at a time while it holds its breath.

Fish also rest. A sleeping fish slows its activity, and its gills pump less. As of November 2026, scientists think most fish have sleep cycles like ours, and zebrafish showed what looked like REM sleep.

Insects are harder to judge. Scientists decide an insect is asleep when it is still and harder to wake, but telling sleep from a sleep-like state is not always easy. Fruit flies sleep, and a fruit fly kept from sleeping needs more sleep afterward, as people do. Fruit flies kept awake for 12 hours chose warmer temperatures afterward, a change scientists used as a sign of disrupted sleep. Honeybees kept awake at night did worse at the waggle dance that tells hive mates where food is. They were less able to signal direction, though their signal for distance was not significantly affected.

Even upside-down jellyfish rest. They are less active at night, pulsing less, and their night-time rest is quickly reversible, which scientists call sleep-like. In 2017 Caltech scientists reported that these jellyfish, which have no centralized nervous system, meet the criteria for a sleeplike state. Scientists also screened 3,683 chemicals on the tiny worm C. elegans and found 42 that strongly reduced its sleep.

Winter rest can resemble sleep, but hibernation is better understood as a state called torpor. Black bears keep their body temperature around 30 degrees Celsius, while their metabolism, which is how fast the body uses energy, falls by up to half. A hibernating ground squirrel goes deeper, and it can drop its metabolic rate by up to 90 percent.

Wood frogs survive winter in a way that looks like the opposite of sleep. Their bodies freeze solid and enter suspended animation: the heart stops beating and breathing ceases, and in spring the heart starts again. How does a frozen frog avoid injury? When ice forms, it pulls water out of cells, and cells that dry out eventually die. The wood frog's liver makes a lot of glucose, a sugar that acts as natural antifreeze and helps keep water inside the cells.

An octopus has quiet sleep and active sleep. Active sleep comes about every half hour and lasts roughly 40 seconds, while its skin flashes changing colors. Scientists compare it to REM sleep in people, though they cannot say whether octopuses dream. So whether an animal sleeps depends on what you count as sleep: quiet, reacting less, and sleeping more after being kept awake. Yet some animals never show a state that meets the behavioral definition of sleep.

Words to know

• electrodes: sensor pads
• unihemispheric: one half-brain
• torpor: deep rest
• metabolism: body's energy use