How Can a Whale Sleep Without Forgetting to Breathe?

Humans continue breathing automatically while asleep. Whales and dolphins face a different problem because every breath must be taken at the water's surface.

These animals are conscious breathers. They must actively control the opening of the blowhole and coordinate each breath with surfacing. Completely losing awareness for hours could therefore be fatal.

Many dolphins and some whales solve the problem with a form of rest called unihemispheric sleep. One half of the brain enters a sleep state while the other remains more alert. After a period, the two sides can exchange roles.

The active half helps control breathing, movement, and awareness of the surroundings. Dolphins resting this way may continue swimming slowly, surface regularly, and keep one eye open. The open eye is generally connected more strongly with the opposite, more active side of the brain.

Whale species do not all rest in precisely the same way. Some remain near the surface, moving gently or lying almost still in a behavior called logging. Sperm whales have been observed resting vertically beneath the surface in groups, rising when they need to breathe. The details and duration of sleep vary across species and remain difficult to study in the open ocean.

Calves create an additional challenge. Newborn dolphins and their mothers may remain highly active during the first weeks of life, when the calf must surface frequently and stay close to its mother.

A whale therefore does not simply hold its breath through an ordinary night of unconscious sleep. Its rest is divided and adapted to an animal that can never surrender completely to the water around it.