How Do Dolphins Sleep? Half-Brain Sleep Explained

What is half-brain sleep?

Scientists call it unihemispheric slow-wave sleep. When Russian researchers recorded the brain waves of bottlenose dolphins in the 1970s, they found that one side of the brain showed the slow, deep waves of sleep while the other side showed the fast activity of an awake brain. After a while the two halves switched roles. Over a whole day, both halves end up with roughly equal amounts of sleep.

Half-brain sleep has since been recorded in other toothed whales, including harbor porpoises, belugas and orcas. Some eared seals use it when sleeping in water, and many birds use it to stay alert to predators. What makes dolphins unusual is that they seem to rely on it almost entirely, rarely letting both halves of the brain sleep deeply at the same time.

Why can’t dolphins sleep like we do?

The main reason is breathing. Dolphins are voluntary, or conscious, breathers: every breath is a deliberate act of surfacing and opening the blowhole, which stays closed by default. A person can breathe automatically in deep sleep, but a fully unconscious dolphin could not come up for air. The waking half of the brain keeps that routine going.

There are other advantages too. An alert half can watch for sharks, which hunt dolphins, keep the animal close to its group and its calf, and keep the body moving. Moving helps a mammal living in water hold on to body heat, which may be another reason dolphins keep swimming slowly as they rest.

Sleeping with one eye open

The nerves from each eye cross to the opposite side of the brain, so when the left half of the brain sleeps, the right eye usually closes, and the left eye stays open. The link is not perfect, and dolphins also switch eyes as the brain halves take turns.

The open eye is often used to keep track of companions. In studies of captive Pacific white-sided dolphins swimming together, animals on the left side of a group tended to keep their right eye open, and those on the right their left eye, so that every dolphin was watching the others rather than the open water. Staying together appears to matter as much as watching for danger.

What sleeping dolphins look like

Resting dolphins are easy to miss. One common posture is logging, in which a dolphin hangs motionless just below or at the surface like a floating log, with its blowhole clear of the water. Another is slow swimming, in which a group travels side by side in a tight formation, moving slowly and breathing at a steady, regular rhythm. NOAA describes resting bottlenose dolphins in just this way.

Some captive bottlenose dolphins also rest on the bottom of a shallow pool for a few minutes at a time, rising to breathe. Wild dolphins rest at different times depending on when and where they feed. Spinner dolphins in Hawaii, for example, hunt offshore at night and rest by day in shallow bays, which is why a US rule since 2021 bans swimming with or approaching them within 50 yards (about 46 m).

How long do dolphins sleep?

Studies of captive bottlenose dolphins found that they spend roughly a third of each day, around 8 hours, in half-brain sleep, spread over many bouts lasting from minutes to a couple of hours. In captivity they rest mostly at night, but wild dolphins fit their rest around feeding, tides and the movements of their prey.

Dolphins can also stay alert for remarkably long periods. In a 2012 study by US Navy researchers, two bottlenose dolphins kept detecting targets with echolocation for 15 days in a row with no sign that their performance was slipping. Half-brain sleep is the most likely explanation.

One thing dolphins seem to lack is clear REM sleep, the stage linked with vivid dreaming in humans. Researchers have found little or no convincing REM sleep in dolphins, although a few studies have reported brief episodes that might be a form of it.

Do mothers and baby dolphins sleep?

Newborn calves must surface often and stay close to their mothers, swimming in the mother’s slipstream. A 2005 study of captive orcas and bottlenose dolphins reported that mothers and newborn calves showed almost none of the typical floating rest for the first weeks after birth, staying on the move around the clock.

Later studies suggested that mothers and calves still rest during this time, but while swimming, closing one eye at a time instead of floating still. Scientists still debate how much sleep they really get, but it is clear that dolphin calves do not need long, still sleep the way human babies do.

Dolphins compared with whales and other animals

Not all whales sleep the way bottlenose dolphins do. Sperm whales have been seen taking short naps while hanging vertically near the surface; see How do whales sleep? Fur seals switch between both-brain sleep on land and half-brain sleep in the water. Large land mammals have their own tricks, from horses that doze standing up to giraffes that sleep in very short spells; see How do giraffes sleep?

To learn more about the species behind most dolphin sleep research, see our profile of the common bottlenose dolphin.