How Animals Sleep

Animal Sleep: Every animal carefully studied shows some form of sleep, even brainless jellyfish, but the ways they do it vary wildly, from sleeping with half the brain to napping in seconds-long bursts while flying or diving.

Great Frigatebird: A 2016 study that recorded its brainwaves found it sleeps in the air, often in bursts of about 10 seconds and sometimes with half its brain, totaling less than an hour a day during flights that last weeks.

Common Swift: Researchers found that some swifts stay airborne for about 10 months at a time, meaning whatever sleep they get must happen on the wing.

Bottlenose Dolphin: It sleeps with one half of its brain at a time, keeping one eye open to watch for predators and stay close to its group while it keeps swimming and surfacing to breathe.

Mallard: In a 1999 study, ducks at the end of a sleeping row kept the eye facing outward open and that side of the brain awake, while ducks in the middle slept more deeply.

Cassiopea: In 2017 researchers showed this upside-down jellyfish, which has no brain at all, enters a sleep-like state at night, suggesting sleep evolved before brains did.

African Elephant: Tracked wild elephants slept only about two hours a day, the least of any mammal recorded, and sometimes went around 46 hours without sleeping.

Little Brown Bat: It is among the sleepiest mammals, sleeping around 19 to 20 hours a day and waking mainly at dusk when insects are most active.

Koala: It spends up to 18 to 22 hours a day sleeping or resting, largely because its eucalyptus diet provides very little energy.

Giraffe: It can get by on remarkably little sleep and often dozes standing up, but for deep sleep it may lie down and curl its long neck back to rest its head on its body.

Stay Apparatus: This system of tendons and ligaments lets horses lock their legs and doze standing up, but they still need to lie down for REM sleep.

Northern Elephant Seal: A 2023 study found that at sea it sleeps about two hours a day in short naps during deep dives, sometimes spiraling downward like a falling leaf while asleep.

Northern Fur Seal: On land it sleeps with both halves of its brain like other mammals, but in water it switches to half-brain sleep and REM sleep nearly disappears.

Sea Otter: Otters often wrap themselves in kelp or hold paws with each other while they sleep on the surface so they don't drift away from their group.

Sperm Whale: Scientists discovered that groups of these whales sleep hanging vertically near the surface, motionless for minutes at a time, for only a small fraction of each day.

Walrus: In captivity walruses have gone more than three days without sleeping, and in the wild they can nap while floating or even while hanging from ice by their tusks.

Orca: Newborn killer whales and their mothers were observed to go without typical sleep for weeks after birth, staying active around the clock.

Chinstrap Penguin: While guarding eggs, these penguins take more than 10,000 microsleeps a day, each lasting only about four seconds, adding up to around 11 hours.

Bearded Dragon: A 2016 study found this lizard cycles between REM-like and slow-wave sleep roughly every 80 seconds, suggesting the building blocks of mammal sleep appeared in reptiles too.

Octopus Insularis: When sleeping, this octopus sometimes flashes through vivid color changes and twitches in an 'active' phase that some scientists compare to dreaming.

Parrotfish: Some species wrap themselves in a mucus bubble each night, a slimy sleeping bag that helps protect them from blood-sucking parasites.

Flamingo: A 2017 study found that a flamingo's body can stay balanced on one leg almost passively, which may be why it can sleep standing on one leg without toppling.

Drosophila Melanogaster: Fruit flies sleep, can be sleep-deprived and then catch up on sleep, which has made them a key lab model for discovering genes that control sleep.

Caenorhabditis Elegans: This tiny worm enters a sleep-like state called lethargus during each growth stage and after stress, showing how deeply rooted sleep is in animal biology.

Arctic Ground Squirrel: Hibernation is not the same as sleep; these squirrels periodically warm up from their deep chill during hibernation, and one theory is that they need to do so in order to sleep.