Rhea Pannett, of Heriot Primary School, asks :-

How do dolphins sleep?

Adrian Leake, a marine zoologist at Napier's Marineland, responded.

Sleep is as important to dolphins as it is to people, a time when we rest our muscles and brain. When we fall asleep our breathing carries on automatically, but with dolphins they have to think to breathe. Dolphins can not stay underwater for too long as they would drown, so it is important that they come to the surface to replenish the air in their lungs. When dolphins sleep it is believed they rest only one half of their brain, the other half remains active to keep up the essential function of breathing.

Some dolphins are what we call "surface sleepers", others are "bottom sleepers". The "surface sleepers", sleep suspended just below the surface. The "bottom sleepers" are river dolphins that sleep on the riverbed. Dolphins do not seem to need as much sleep as we do because dolphins are able to float in their aquatic environment. We use our muscles all day as we sit, stand, walk and run, with the force of gravity working on our bodies. With the dolphin's ability to float they are able to rest their muscles even if they are not sleeping.

Sleep would also be limited by the threat of predators, so dolphins in their social groups would need to be on guard for any potential dangers.