How do cetaceans (dolphins, whales) sleep?

How can an animal that has to breathe voluntarily at the surface fall asleep out at sea without drowning? This question, simple in appearance, puzzled biologists for a long time. The answer is one of the most surprising adaptations in the animal kingdom: cetaceans never sleep entirely. They have developed a form of sleep that allows them to recover mentally while continuing to breathe, to watch their surroundings and to swim.

Unihemispheric sleep: sleeping on one side of the brain

The mechanism is called unihemispheric slow-wave sleep. At any given moment only one half of the cetacean’s brain comes to rest, in deep sleep with the characteristic slow brain waves. The other half stays fully active, with the fast waves typical of wakefulness. The phenomenon can be seen: the eye on the side of the sleeping hemisphere closes, while the opposite eye stays open and goes on watching the surroundings.

The two hemispheres alternate their rest phases. In the common bottlenose dolphin, for instance, each side of the brain sleeps for about an hour before handing over to the other. Over 24 hours the animal accumulates between four and eight hours of sleep in all, but never on both hemispheres at once.

Why this is necessary

The main reason is respiratory. Unlike land mammals, for which breathing is an automatic reflex, cetaceans take each breath in a voluntary and conscious way. If their whole brain fell into deep sleep, they would stop returning to the surface and would drown within minutes. The hemisphere that stays awake handles this vital function: the decision to surface, control of the swimming muscles, opening of the blowhole.

The other reason is vigilance towards predators. Dolphins and porpoises are potential prey for killer whales, large sharks and certain other cetaceans. Keeping one eye open and one cerebral hemisphere on watch makes it possible to detect a threat and react quickly, even while resting. In species that live in groups, individuals are often observed sleeping in turn, with those awake placed as sentinels around the core of the group.

Logging and the other sleeping postures

During unihemispheric sleep, cetaceans adopt several postures depending on the species. The best known is logging (biotage in French): the animal floats motionless just below the surface, like a log of wood (hence the name), with only the top of the head and the blowhole out of the water. It comes up to breathe from time to time with a simple beat of the tail, almost without awareness.

In other species, sleep goes with slow, steady swimming: the animal keeps moving in lazy circles, at low speed, while asleep. This is the case in particular for sperm whales, which often sleep in vertical groups, head down or head up, hanging motionless in midwater like bottles floating upside down.

Newborn calves and their mothers

During the first weeks of life, the calf and its mother sleep almost not at all. This behaviour, studied in particular in common bottlenose dolphins in captivity, is unique among mammals: while most young mammals sleep a great deal, newborn cetaceans stay awake continuously for a month or more, accumulating only very short periods of rest. The mother falls in with this rhythm. The leading hypothesis is that this permanent vigilance makes up for the lack of insulating blubber in the calf, which has to swim constantly so as not to cool down.

An adaptation unique to the aquatic world

Unihemispheric sleep is not confined to cetaceans. It is found, to varying degrees, in eared seals, in certain migratory birds (which can thereby fly for days without a break), and even in certain reptiles. But nowhere is it as developed and as systematic as in cetaceans, which attests to the intense evolutionary pressure bearing on a mammal that returns to aquatic life without being able to breathe there automatically.

---

Further reading