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Do Dolphins and Whales  Sleep?

It’s hard to imagine a whale or dolphin sleeping. Do they simply float at the surface, completely unaware of their surroundings? Picture the open ocean. It’s big, really, really big, and theres literally no where to hide. So how can they protect themselves while they’re asleep?

It turns out that cetaceans are capable of sleeping without becoming fully unconscious, unlike you and I. By shutting down only one hemisphere of their brain at a time they are always partially alert. This process, termed unihemispheric slow-wave sleep, is also displayed in many species of birds and seals. While one half of their brain is resting, the other half is looking around and monitoring their breathing.

It can still be difficult to tell whether a cetacean is asleep. Normally individuals will remain motionless at the surface but some will continue to swim slowly. There is, however, one small indication. If you’re lucky enough to get a closer look at a sleeping whale or dolphin you may notice that one eye will always remain open. This “winking” is caused by the opposite half of the brain. If the left side is awake, the right eye will be open and visa versa.

Although this is very useful for cetaceans, they are also confronted with another risk while sleeping: boats. Pilot whales, for example, are nocturnal, making them an extremely easy target for whale watching boats and eager tourists. But is it fair to approach them while they are trying to rest? How would you feel if you were constantly disturbed while you tried to sleep?

By Caitlyn Skelton - Tenerife Assistant Research Officer

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