Do animals sleep? They do, but how much they need varies enormously, and some seem to manage on remarkably little. Male lions sleep nearly 20 hours a day, but elephants sleep only about 2 to 3 hours a day, and 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, while wild matriarchs lay down to sleep only every third or fourth day. After a night without sleep they showed no sleep rebound, which means that they did not appear to make up for what they had lost.
You may have heard that dolphins never sleep, but in fact dolphins are known for unihemispheric sleep, in which half the brain sleeps while the other half stays active. Birds can sleep in a similar way, and a bird that sleeps with half its brain closes only the eye on the sleeping side. Adult orcas normally snooze 5 to 8 hours a night, closing both eyes and floating motionlessly. Newborn killer whales and dolphins are a different case, because for the first 30 days after birth neither the calf nor its mother shuts its eyes, and over the following months both gradually rest more until they reach normal adult levels. Northern fur seals also sleep with half a brain. 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 a fur seal can even briefly open one eye while a recording shows that one half of its brain is asleep. The brain chemical acetylcholine is released unevenly during half-brain sleep, with much more on the awake side.
Frigatebirds face an even harder problem, because they may have to sleep while flying. Scientists tracked great frigatebirds on flights lasting up to ten days and 3000 kilometers, attaching a small recorder to each bird's head that registered the activity of both brain halves. The recordings showed that the birds can sleep in flight, using one half of the brain at a time or both halves together, but in flight they slept about 0.69 hours a day, which is just 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.
Other birds raise the same question about sleeping on the wing. Alpine swifts may hardly ever land, and scientists who tagged six of them with small recorders of light and movement found that the birds stayed in the air for about 200 days, from their trip to Africa through the winter. Common swifts, tracked with 19 lightweight data loggers, spent about ten months of the year almost constantly in the air, with at most 0.64 percent of migration spent on the ground. If these birds sleep during such trips, they must do it while flying.
Elephant seals show how one 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 of about 30 minutes they take naps of about 10 minutes at a time while holding their breath. 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.
Where scientists measure sleep matters. In the rainforest, wild three-toed sloths slept about 9.6 hours a day, while older studies of sloths kept in captivity had said about 16 hours. To measure wild sloths, scientists fitted them with a small, lightweight cap loaded with brain-activity sensors and a radio collar, a technique first made for people and adapted for animals living freely in the forest. Captive sloths may have been more relaxed, with no threats from predators and food always supplied, and the scientists warn that measuring brain activity only in captivity can lead to the wrong conclusions about wild animals.
How can anyone be sure that an animal is asleep? Scientists call a rest sleep when the animal is quiet, reacts less to things around it and sleeps more after being kept awake. A sleeping ostrich sits still with its eyes open and its neck upright, so it looks alert. To measure its sleep, scientists put helmets with electrodes on six ostriches to measure body temperature, brain activity, eye movements and neck muscle contractions. Sperm whales floating upright were called asleep using a standard test rather than brain waves, which looks for reduced responsiveness that reverses with enough stimulation to cause waking. Researchers brought a boat near floating whales, and the whales did not react until it accidentally touched one. Researchers have said that measuring the whales' brain waves at sea is a challenge for future technology.
Some sleep is very brief. An octopus cycles through two kinds of sleep: in quiet sleep it is pale and motionless with its pupils narrowed to slits, while in active sleep its skin flashes changing colors and textures and its eyes, arms and suckers move. Active sleep is rare, coming about every half hour and lasting roughly 40 seconds, so it takes up less than 1 percent of an octopus's day. One scientist suggested that if dreaming happens in this state, it should be more like small video clips, or even gifs, because the state lasts only up to a minute. In the cuttlefish, a state like REM sleep repeats in cycles, each lasting about 2.4 minutes and alternating with about 34 minutes of quiet rest.
Hibernators are not all alike. Black bears spend about six months a year denning, curled up without eating, drinking or urinating, and their body temperature stays around 30 degrees Celsius, while ground squirrels dip below freezing and tenrecs keep theirs around 32 or 33 degrees Celsius. A dwarf lemur does not keep a steady body temperature while hibernating: its body temperature can swing by as much as 36 degrees Fahrenheit, depending on how well its tree hole is insulated.
Is hibernation just a very long sleep? It is better understood as a state called torpor, and sleep, torpor and hibernation are three different low-energy states that share slowed heart rate and lower body temperature. 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, whereas a hibernating ground squirrel can drop its metabolic rate by up to 90 percent. One 1984 paper argued that sleep, shallow torpor and hibernation are related energy-saving states on a continuum of falling metabolism and body temperature. Hibernators also wake on a schedule: depending on the squirrel, torpor is interrupted every 14 to 21 days, when the body warms to about 37 degrees Celsius and the squirrel stretches and then goes back to sleep.
What happens when sleep is taken away? A fruit fly kept from sleeping needs more sleep afterward, as people do, and flies kept awake for 12 hours chose warmer temperatures afterward, a change scientists used as a sign of disrupted sleep. In honeybees, researchers devised a method to keep some bees awake without disturbing the rest of the hive, and the sleepless bees did worse at the waggle dance that tells hive mates where food is, signaling direction, though not at signaling distance. Scientists decide an insect is asleep when it is still and harder to wake, although telling sleep from a sleep-like state is not always easy.
Tiny worms give more clues. In C. elegans worms, a sleep neuron called RIS and its signal FLP-11 are required for memory consolidation, which means storing a memory. Disrupting developmental sleep, which means sleep early in life, was linked to more feeding and much more lipid build-up, which means fat. Scientists also screened 3,683 chemicals on this worm and found 42 that strongly reduced its sleep.
Jellyfish rest too. Upside-down jellyfish are less active at night, pulsing less. The jellyfish Cassiopea shows a night-time rest that is quickly reversible, which scientists call sleep-like. In 2017 Caltech scientists reported that upside-down jellyfish, which have no centralized nervous system, meet the criteria for a sleeplike state.
In one study of male pectoral sandpipers in the Arctic summer, the males that slept the least sired the most young, and the study calls this the first evidence of adaptive sleep loss, which challenges the view that losing sleep always hurts. Males mate with several females and must constantly repel rivals, and because the sun never sets, males that can keep up this extreme competition have more chances to mate. Short sleeping did not seem to cost them their lives, since successful males returned to the breeding area more often than males that sired fewer young. The males that slept the least also had the deepest sleep when they did sleep.
So can sleep be cheated? The sandpiper findings come with a caveat, because the birds that slept least were still experiencing a deficit in sleep, and one suggested explanation is that long-sleeping males may lack genetic traits that enable short-sleeping individuals to maintain high performance. A 108-hour total sleep deprivation did not make northern fur seals worse at a visual memory-matching task. As of November 2026, scientists say that some species routinely sleep little, or can cut their sleep for a while, seemingly without cost, but whether any cost exists is still unclear.
Words to know
- matriarchs: lead females
- rebound: catch-up sleep
- paralysis: unable to move
- torpor: deep rest
- caveat: a warning
