How Do Whales Sleep?

Whales, like all mammals, need sleep in order to survive. But they are also air breathers, meaning they cannot become fully unconscious while in the water.

There are around 90 different types of whale, which can hold their breath for around an hour or so, depending on the species.

However, they usually travel up to the surface to take a breath out their blowhole every 15 minutes, and as such are never fully submerged in the water for very long.

So, how exactly do they get any sleep? The short answer is, "very differently" from other types of mammal.

Naomi Rose, Marine Mammal Scientist at the Animal Welfare Institute, told Newsweek that as a general rule of thumb, cetaceans—which are the family of aquatic mammals including whales, dolphins and porpoises—cannot become fully unconscious or else they would drown.

"It's an interesting dilemma for wholly aquatic air-breathers. To deal with living in the water full-time, while having to breathe air at the surface, they have evolved into voluntary breathers, as a way to prevent accidentally inhaling water at inopportune moments," Rose said.

Rose said whales consciously control their blowholes with "powerful muscles," meaning they have to be awake and alert at all times to prevent themselves from drowning.

"They do not breathe autonomously, as terrestrial animals do," Rose said. "If they were unconscious, which means being fully asleep, they would not breathe and would drown. So [whales] have solved the problem with unihemispheric sleep: that is, they shut down only one half of the brain at a time, keeping one-half conscious and breathing."

Whales have some of the largest brains on the planet. Sperm whales and killer whales in particular have the biggest brain of any living mammal. This means they can actively decide which part of their brains to use at a given time.

Rose said this peculiar way of sleeping can be seen most clearly in captive whales, as they are easier to see. When whales are "sleeping" they can be seen keeping one eye closed while the other remains open.

"The behavioral state is in fact known as resting, rather than sleeping, for this reason. They continue to swim, slowly and regularly—in tight synchronous formation for social cetaceans—occasionally floating still for a few seconds, up to a couple of minutes, perhaps, often very near the surface," she said.

This sleeping technique varies slightly between species, however. Rose said that some species, like sperm whales, enter a deeper sleep where they hang in groups, vertically, not too far below the surface for just over an hour before they surface to breathe.

Other species, such as the humpback whales, have been observed resting motionless at the surface of the water for increments of only 30 minutes. Humpbacks cannot sleep for much longer than this without losing too much of their body temperature.

And, killer whales, who are very socially complex, never stray too far from the other members of their pod, sleeping in tight-knit groups.