Klaus Esterluss

Deutsche Welle

It must be a slightly surreal sight. Imagine being a diver and coming across a group of sperm whales, not swimming but floating upright and motionless in the open sea. If you ever see such a scene, congrats, you have found cetaceans sleeping. It's a rare occurrence, but occasionally pictures of such discoveries are published.



There are even researchers working on the subject on different species of marine mammals - both in the wild or in captivity. The results of both approaches allow an impression of how sleep works in a whale's world.



Back in 2008, a team of scientists from Scotland and Japan literally bumped into a group of sperm whales drifting vertically close to the surface of the water. The mammals were so immersed in a state called "drift diving" that they didn't even respond to the presence of the passing boat. By observing the whales, the researchers onboard found they spend just 7% of their day napping, for bursts of around 10 to 15 minutes each time.

Half-awake but sleeping



Such modest slumber make sperm whales the species least needy of sleep on the planet, dethroning giraffes who spend some 8% of their day in slumber. Humans would likely not be able to live like that. When we sleep, we do so for several hours in a row. The reason is that we are involuntary breathers, which means we can count on our ability to breathe automatically.



Whales, on the other hand, have to think about every breath they take. Scientists believe they sleep with one eye open and one half of their brain awake, not only to control their breathing but also to ensure they can avoid predators, maintain social contact or continue swimming. The phenomenon is called unihemispheric slow-wave sleep and has also been recorded in whales living in captivity.