Cetacean sleep: sleeping with half a brain

How do you sleep when every breath has to be a conscious act? Cetaceans have resolved this paradox through unihemispheric sleep: only one hemisphere of the brain sleeps at a time, the opposite eye staying open. They can thus keep swimming, come up to breathe and watch for predators.

The widget above explores this mechanism and the most surprising observations: sperm whales sleeping vertically in groups, or newborn killer whales that barely sleep at all during their first weeks of life.

A sleep shared with birds and sea lions

Unihemispheric sleep is not unique to cetaceans. It is also seen in many birds (which sleep with one eye open to watch for predators or to stay on the wing for long periods) and in sea lions. Everywhere it answers the same requirement: staying partly alert while recovering.

In cetaceans, this mode of sleep is imposed by voluntary breathing and by the absence of any support: they cannot stay still without sinking or without stopping breathing. The near-absence of REM sleep (paradoxical sleep) follows from this, the muscular atonia associated with dreaming being incompatible with aquatic life.

Going further: discover the physiology of diving, the brain and cognition and the senses of cetaceans.