You may have heard that dolphins never sleep. In fact, a dolphin rests one half of its brain while the other half stays awake, a pattern called unihemispheric sleep. Northern fur seals show it clearly: during one kind of sleep, called slow-wave sleep, their brain waves are strongly lopsided, with one half of the brain more asleep than the other, and the brain chemical acetylcholine is released unevenly during this half-brain sleep, with much more on the awake side. A fur seal can even briefly open one eye while a recording shows one half of its brain asleep. Some birds sleep with half a brain too, and a bird that does this closes only the eye on the sleeping side.

Great frigatebirds put half-brain sleep to a hard test. Scientists tracked them on flights lasting up to ten days and 3000 kilometers, with a small recorder on 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 they slept only 0.69 hours a day, which is 7.4 percent of the time they spent asleep on land. The study suggests that the demands of staying alert usually exceed the attention that half-brain sleep allows, and it challenges the idea that birds make long flights while getting normal amounts of sleep on the wing.

Skipping sleep can even pay off. In one study of male pectoral sandpipers in the Arctic summer, the males that slept the least sired the most young, which the study calls the first evidence of adaptive sleep loss. It challenges the view that losing sleep always hurts. The males mate with several females and must constantly repel rivals, and the sun never sets during the Arctic summer, so males that can keep up this extreme competition have more chances to mate. Short sleeping did not seem to cost the sandpipers their lives. Successful males returned to the breeding area more often than males that sired fewer young, and the males that slept the least also had the deepest sleep when they did sleep. Northern fur seals add to the puzzle: a 108-hour total sleep deprivation did not make them worse at a visual memory-matching task.

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, and wild matriarchs lay down to sleep only every third or fourth day.

Elephant seals show how the same animal can change its sleep with its surroundings. Scientists recorded brain activity in wild seals with a watertight cap and found that they averaged just 2 hours of sleep a day at sea, compared with 10 to 11 hours on land. On deep dives they nap while holding their breath, in naps of about 10 minutes during dives of about 30 minutes. Diving elephant seals enter slow-wave sleep while gliding down, then REM sleep, when sleep paralysis makes them turn upside down and drift in a sleep spiral.

Winter dormancy is another way to spend less. Hibernators are not all alike, since bears keep their body temperature around 30 degrees Celsius, ground squirrels dip below freezing, and tenrecs, in contrast, want their body temperatures around 32 or 33 degrees Celsius. In an Alaska study, hibernating black bears functioned at only 25 percent of their active metabolic rate even though their body temperature did not drop much. A hibernating ground squirrel can drop its metabolic rate by up to 90 percent. Hibernators also wake on a schedule, and depending on the squirrel, the torpor is interrupted every 14 to 21 days.

Is hibernation just a very long sleep? Scientists say it is better understood as a state called torpor. Sleep, torpor and hibernation are three different low-energy states that share slowed heart rate and lower body temperature. One 1984 paper argued that sleep, shallow torpor and hibernation are related energy-saving states on a continuum of falling metabolism and body temperature. The build-up of adenosine seems to be common before entry into sleep, torpor and hibernation. Hummingbirds also use torpor. As of November 2026, scientists report that on a very cold night, a hummingbird's metabolism can drop by as much as 95 percent.

Which animals have REM sleep is argued over, and a 2008 review found that convincing evidence of REM sleep in reptiles, fish and insects was lacking, and some marine mammals show no REM sleep. Yet bearded dragons, which are reptiles, show both slow-wave and REM sleep, two kinds of sleep that last about equally in each cycle. In mammals, REM is much shorter than slow-wave sleep, and in birds, REM is both short and irregular. As of November 2026, scientists think most fish have sleep cycles like ours, and zebrafish showed what looked like REM.

Small animals also seem to sleep. A fruit fly kept from sleeping needs more sleep afterward, as people do, and flies kept awake for 12 hours chose warmer temperatures, a change scientists used as a sign of disrupted sleep. 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. Upside-down jellyfish are less active at night and pulse less, and in 2017 Caltech scientists reported that these jellyfish, which have no centralized nervous system, meet the criteria for a sleeplike state.

In the end, the answer depends on the test. Sperm whales float upright, almost motionless, with their heads at or just below the surface of the sea. Researchers approached floating whales in a boat, and the whales did not react, even though the boat was visible to both eyes, until it accidentally touched one. The whales were called asleep by a standard test, not by brain waves: reduced responsiveness that reverses with enough stimulation to cause waking. Scientists call a rest sleep when the animal is quiet, reacts less to things around it and sleeps more after being kept awake. Yet some animals never show a state that meets the behavioral definition of sleep.

Words to know

• unihemispheric: one half-brain
• acetylcholine: a brain chemical
• adenosine: a body chemical
• torpor: deep rest
• matriarchs: lead females