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. Fruit flies meet that last test, because a fruit fly kept from sleeping needs more sleep afterward, as people do. Yet some animals never show a state that meets the behavioral definition of sleep.
Insects and jellyfish test the idea. 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, and fruit flies kept awake for 12 hours chose warmer temperatures afterward, a change that researchers 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, and the sleepless bees did worse at signaling direction, though not at signaling distance. 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.
Half-brain sleep lets an animal rest while part of the brain stays awake. Dolphins are known for unihemispheric sleep, in which half the brain sleeps while the other half stays active, and northern fur seals show a strongly lopsided brain-wave pattern during one kind of sleep, with one half of the brain more asleep than the other. The brain chemical acetylcholine is released unevenly during this sleep, with much more on the awake side, and a fur seal can even briefly open one eye while a recording shows one half of its brain asleep. Birds that sleep with half a brain close only one eye.
Great frigatebirds were tracked 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 about 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.
Swifts raise the same question about sleeping on the wing. Alpine swifts stayed in the air for about 200 days, from their trip to Africa through the winter, and if they sleep during that time they must do it while flying. Common swifts, tracked with 19 lightweight data loggers that recorded light, acceleration and body position, 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.
Elephants sleep only about 2 to 3 hours a day. 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 wild matriarch went without sleep for up to 46 hours, and wild matriarchs showed no sleep rebound after a night without sleep.
Sandpipers and fur seals add to the puzzle. 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. The birds that slept least were still experiencing a deficit in sleep, though. 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.
Diving mammals have to sleep in a very different place. Scientists measured sleep in wild elephant seals by recording brain activity with a watertight cap, and they found that at sea the seals averaged just 2 hours of sleep a day, compared with 10 to 11 hours a day on land. On dives of about 30 minutes they nap for about 10 minutes at a time while holding their breath, entering slow-wave sleep while gliding down and then REM sleep, when sleep paralysis makes them turn upside down and drift in a sleep spiral.
Sperm whales sleep upright, in groups, floating vertically and almost motionless with their heads at or just below the surface. Researchers brought a boat near floating whales, and the whales did not react until it accidentally touched one. They were called asleep using a standard test, not brain waves: reduced responsiveness that reverses with enough stimulation to cause waking.
Fish need to rest, too, and a sleeping fish slows its activity while its gills pump less. Sharks do not always have to keep swimming to sleep: many slow down or swim into a current, and as of November 2026, scientists think most fish have sleep cycles like ours, with zebrafish showing what looked like REM sleep.
Hibernation is often described as a very long sleep, but 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, and 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 are not all alike, since bears keep their body temperature around 30 degrees Celsius, ground squirrels dip below freezing, and tenrecs keep theirs around 32 or 33 degrees Celsius.
Wood frogs do something that looks like the opposite of sleep. Their bodies freeze solid and enter suspended animation, with the heart stopping and breathing ceasing, and in spring the heart starts again. When ice forms, it pulls water out of cells, and cells that dry out eventually die, so 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 alternates between quiet sleep, when it is pale and motionless with its pupils narrowed to slits, and active sleep, when its skin flashes changing colors and textures and its eyes, arms and suckers move. Active sleep is rare and brief, coming about every half hour, lasting roughly 40 seconds and taking up less than 1 percent of an octopus's day. 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 sleep-like rest. One scientist suggested that if octopuses dream, their dreams should be more like small video clips, or even gifs, because the state lasts only up to a minute. Octopuses shown crab pictures and vibrations responded slowly or not at all, which suggested they were asleep.
Reptiles complicate the simple story that REM sleep belongs to birds and mammals. 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 show both slow-wave and REM sleep, and scientists once thought these patterns belonged only to mammals and birds. In mammals REM is much shorter than slow-wave sleep, but in bearded dragons the two states last about equally in each cycle, and a bearded dragon's cycle is about 80 seconds long at 27 degrees Celsius, compared with about 30 minutes in cats and 60 to 90 minutes in humans. Researchers wrote that reptile sleep seems a lot simpler than mammal sleep, a stripped-down version of the richer range of mammal sleep, and may thus be closer to the ancestral mode of brain sleep.
Birds add another clue about how sleep began. A sleeping ostrich can fool an observer by sitting still with its eyes open and its neck upright, so to record its sleep, scientists put helmets with electrodes on six ostriches that measured body temperature, brain activity, eye movements and neck muscle contractions. During REM periods, an ostrich's brain flips between fast, shallow waves and slow waves, which is reminiscent of the platypus and echidna. The scientists say that the two distinct sleep states, slow-wave and REM, arose from a single mixed state, and that REM sleep represents an evolutionarily new feature, and the Sentinel Sleep Theory proposes a possible job for it: REM sleep evolved to heighten alertness and offset the vulnerability of non-REM sleep.
What might sleep be for? A 2026 paper reported that DNA damaged while awake is repaired during sleep, which would be an evolutionarily ancient function. In the tiny worm C. elegans, a sleep neuron called RIS and its signal FLP-11 are required for memory consolidation, which means storing a memory, and disrupting developmental sleep, which means sleep early in life, was linked to more feeding and much more lipid build-up, which means fat. The build-up of a chemical called adenosine seems to be common before entry into sleep, torpor and hibernation. As of November 2026, scientists note that sleep is widespread across animal species, yet its functions remain enigmatic.
Words to know
- unihemispheric: one half-brain
- torpor: deep rest
- adenosine: a body chemical
- enigmatic: hard to explain
- ancestral: from long ago
