How Do Killer Whales Sleep?

Orcas, also known as killer whales, are large members of the dolphin family, alongside bottlenose dolphins, pilot whales, belugas and more. Like all members of the Delphinidae family, these marine mammals live their entire lives in the water and never spend any time on land. However, if they never leave the water, how do orca whales sleep without drowning?

If they spend their entire lives in the water, exactly how do orca whales sleep? In short, they don't – at least, not fully. Like all other cetaceans (whales and dolphins), orcas do not entirely shut down their brain for sleep the same way that you do. Instead, they sleep only with half of their brain at a time and the other half remains conscious and alert.

The ability to sleep with half of the brain is known as unihemispheric slow-wave sleep. In this form of sleep, the animal sleeps with half of the brain and keeps the other half conscious. During unihemispheric slow-wave sleep, the whale also closes the eye on the side opposite the side of the sleeping brain.

The conscious half of the brain tells the animal's body when it is time to rise to the surface for a breath of air and also keeps watch for predators or other dangers. After a period of about two hours, the animal switches the sleeping side to the other half of the brain so that both halves of the brain are rested.

Orca whales sleep with only half their brains because they must breathe voluntarily to avoid drowning. They have to periodically consciously rise to the surface to breathe. They must also keep their blowhole, the flap of skin that they open and close to breathe, closed at all other times to keep water out. In humans, breathing is involuntary and occurs without conscious thought, but all whales and dolphins must be conscious in order to breathe.

For this reason, orcas generally sleep for short periods of time, taking the equivalent of "catnaps."

Though they typically sleep for short periods, how many hours a day do orcas sleep in total? The answer can vary based on the individual animal as well as major life events. For example, mother killer whales do not sleep or rest at all for the first month of their calf's life. Their period of rest gradually increases from the time their calf is one month old until they reach regular sleeping periods again.

Researchers found that bottlenose dolphins sleep for approximately 33.4 percent of a 24-hour period, which equates to about eight hours total. Killer whales can sleep for short periods but have also been observed sleeping for eight continuous hours in human care.

Continuous swimming can tire an orca calf, and weakened animals are more prone to illness and are also vulnerable to predators. However, because calves do not have a thick layer of blubber when they are first born, they don't float as easily as adult whales do. For this reason, mothers often employ a unique method to help their offspring rest, known as echelon swimming.

In echelon swimming, the calf moves into the mother's slipstream, which carries the calf along with the mother without the calf having to swim. This allows the calf to rest and sleep without having to exert energy to keep swimming. By riding in their mother's slipstream, the calf can take time to rest. However, this does mean that the mother whale cannot stop swimming until her calf has enough blubber to float easily and rest on its own.