How Do Dolphins Sleep? Half a Brain at a Time

We sleep by lying down, closing our eyes and switching off for hours. For a dolphin, that would be fatal. Dolphins are mammals that breathe air, living in water, and they have found a remarkable solution: they sleep with half a brain at a time.

One half sleeps, the other keeps watch

When scientists record the brain waves of sleeping dolphins, they see something unusual. One hemisphere shows the slow, large waves of deep sleep, while the other shows the fast activity of an awake brain. This is called unihemispheric slow-wave sleep.

The eye connected to the awake hemisphere usually stays open, while the other closes. After a period of rest, the two halves swap, so both get the sleep they need. Over a day, the sleep adds up to several hours.

Why dolphins can’t switch off completely

The main reason is breathing. In humans, breathing is automatic; we keep breathing while unconscious. Dolphins breathe voluntarily: each breath is a decision to surface and open the blowhole. If the whole brain fell into deep sleep, the dolphin might not surface and could drown.

Keeping half the brain awake also helps them:

• watch for predators such as sharks;
• stay in contact with their group and their calves;
• keep swimming and maintain body heat in cold water.

A study of bottlenose dolphins and killer whales reported that mothers and newborn calves showed almost no typical sleep behaviour for weeks after birth, staying constantly on the move, which may protect the calf and keep it warm.

Other half-brain sleepers

Dolphins are not alone. Fur seals sleep with half the brain when in water but switch to more normal sleep on land. Many birds do it too. In a classic experiment, ducks sleeping in a row kept the outward-facing eye open, with the matching half of the brain awake, watching for danger, while ducks in the safer middle slept with both halves more often.

In 2016, researchers fitted frigatebirds with tiny brain recorders and showed for the first time that birds really do sleep in flight, sometimes with one hemisphere and sometimes with both, but only for about 40 minutes a day, compared with over 12 hours on land.

Do humans do something similar?

People cannot sleep with only half the brain, but there may be an echo of it. A 2016 study found that on the first night in a new place, one hemisphere of the sleeper’s brain stays more alert, reacting more to unusual sounds and waking more quickly. This “first-night effect” may explain why many people sleep badly in hotels or unfamiliar beds.

In short

• Dolphins sleep with one half of the brain at a time: unihemispheric slow-wave sleep.
• The eye linked to the awake half often stays open.
• They must stay partly awake because they breathe consciously.
• Half-brain sleep also helps them watch for predators and stay with their group.
• Seals, some whales and many birds, including ducks and frigatebirds, sleep this way too.
• Humans show a hint of it on the first night in a new place.