Do animals sleep? They do, and some of them do it in very odd ways. You may have heard that dolphins never sleep. In fact, dolphins are known for a kind of sleep called unihemispheric sleep, in which one half of the brain sleeps while the other half stays active. Northern fur seals show something similar. During slow-wave sleep, their brain waves are strongly lopsided, with one half of the brain more asleep than the other. Some birds sleep this way too, and a bird closes only the eye on the sleeping side.

Can a bird sleep while it flies? To find out, scientists tracked great frigatebirds on flights lasting up to ten days and 3000 kilometers. They attached a small recorder to each bird's head. It registered the activity of both halves of the brain at once.

The recordings showed that great frigatebirds can sleep in flight. They sometimes used one half of the brain at a time and sometimes used both halves together. But the sleep was short: in flight, the birds slept for only about 0.69 hours a day. Alpine swifts may go even longer without landing. Scientists tagged six of them with small recorders of light and movement, and the data showed the birds stayed in the air for about 200 days, from their trip to Africa through the winter.

Elephant seals sleep while they are far from shore, and scientists wanted to measure it. They recorded brain activity in wild elephant seals with a watertight cap. At sea the seals averaged just 2 hours of sleep a day, while on land they slept 10 to 11 hours a day. On deep dives, a seal takes short naps while it holds its breath.

Sperm whales sleep in an odd way, upright. Cameras on tagged whales showed groups of them floating straight up and down, almost motionless, with their heads at or just below the surface of the sea. But looking asleep is not the same as being asleep, so researchers tested the whales. Their boat approached floating whales, and the whales did not react, even though the boat was visible to both eyes, until it accidentally touched one.

Elephants sleep only about 2 to 3 hours a day. Two wild African elephant matriarchs, the lead females of their herds, were fitted with trackers. They slept about 2 hours a day on average, mostly between 2 a.m. and 6 a.m. Giraffes sleep mostly at night but take quick naps through the day. In a zoo study, they slept about 4.6 hours a day on average, and their sleep cycles lasted 35 minutes or less. Giraffes can sleep standing up or lying down.

Hibernation is not just a long sleep. It is better understood as a state called torpor, a state of very low energy use. Bats use torpor to save energy. Black bears spend about six months a year in a den, curled up without eating, drinking or urinating. Their metabolism, which is how fast the body uses energy, can fall by up to half. But unlike ground squirrels, they keep their body temperature around 30 degrees Celsius. A hibernating ground squirrel can drop its metabolic rate by up to 90 percent.

An octopus cycles through two kinds of sleep. In quiet sleep, it is pale and motionless. In active sleep, its skin flashes colors and its eyes, arms and suckers move. Active sleep comes about every half hour and lasts roughly 40 seconds. It takes up less than 1 percent of the octopus's day, and scientists compare it to REM sleep in people, though they cannot say whether octopuses dream.

Fruit flies sleep, and a fly kept from sleeping needs more sleep afterward, as people do. Flies kept awake for 12 hours chose warmer temperatures afterward. Scientists used that change 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 the direction to a feeder, though their signal for the distance was not significantly affected.

Where scientists measure sleep matters. To measure sleep in wild sloths, scientists fitted them with a small, lightweight cap loaded with brain-activity sensors, plus a radio collar. Wild three-toed sloths slept only about 9.6 hours a day. Older studies of sloths kept in captivity had said about 16 hours. Captive sloths may have been more relaxed, with no threats from predators and food always supplied. The scientists warn that measuring brain activity only in captivity can lead to the wrong conclusions about wild animals.

Sleep turns up in tiny worms and jellyfish, too. Scientists screened 3,683 chemicals on the tiny worm C. elegans and found 42 that strongly reduced its sleep. Upside-down jellyfish are less active at night, pulsing less. Their night-time rest is quickly reversible, which scientists call sleep-like. Jellyfish have no centralized nervous system. Yet in 2017 Caltech scientists reported that they meet the criteria for a sleeplike state. So the answer is yes, even for an animal with no centralized nervous system.

Words to know

• unihemispheric: one half-brain
• torpor: deep rest
• matriarchs: lead females
• metabolism: body's energy use