Do Sharks Sleep?

The answer to 'Do Sharks Sleep' is not a straightforward one. Sharks certainly don't seem to go into a deep sleep like humans do but whether they have their own special type of 'shark sleep' is still not scientifically known.

What we do know is this. In order to breathe or obtain oxygen, sharks need to have water flowing or moving over their gills (respiratory organs) which are slits to the side of their heads. Many sharks are continually on the move in order to facilitate this breathing process but some sharks can actually stay motionless and still breathe. Sharks that can lie still have spiracles, small openings behind their eyes that move water across their gills whilst at 'rest.'

In the human definition of sleep, unless we are sleep-walking, we tend to lie still and this uses far less energy than when we are awake and on the move. However, sharks that are still have to work harder to get oxygen moving over their gills so they can breathe. So it's debatable whether a shark lying still is really and truly at rest - certainly for some sharks it requires using up more energy. And just because a shark is still doesn't mean they are asleep as divers and other marine life are well aware!

Even for sharks who need to swim constantly in order to breathe, it is thought that they can have both active and restful periods. Parts of the shark brain may be at rest or less active even whilst they swim - a bit like 'sleep swimming.'

Image Credit: By Duncan Wright via Wikimedia Commons