A dolphin sleeps with one half of the brain

In slow-wave sleep, dolphins put one hemisphere to sleep at a time. The other stays awake, one eye stays open, and the animal surfaces to breathe.

Electrodes on captive dolphins show high-amplitude slow waves on one side of the brain while the other side shows a waking pattern. The eye opposite the sleeping hemisphere is typically closed, and the eye opposite the waking hemisphere is open. The sides switch. Over a day the two hemispheres get similar amounts of sleep, in shifts. During this the animal swims, slowly and often in a circle, and it rises to breathe on schedule. The behaviour and the EEG were recorded together. It is not a guess from an eye being shut.

Birds do a version of the same thing, including on the wing, which is another entry. In dolphins the stake is the breath. They lack the automatic respiratory drive that lets a sleeping human inhale. Unihemispheric sleep is how a mammal can be unconscious enough to restore one half of a brain and conscious enough not to drown. It is not ‘dolphins never sleep.’ They sleep a great deal. They just refuse to do it all at once.