Whales, like humans and all other mammals, need sleep in order to survive. But whales, unlike humans, live in water. But we all need to breathe air; so how do whales manage to sleep?
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 through 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 inhaling water.
"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."
Some migratory birds, that stay airborne for many days at a time, also ultilise unihemispheric sleep.
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."