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Do dolphins sleep?
Dolphins are mammals so cannot breathe underwater. This makes sleep difficult as dolphins can only hold their breath for up to 20 minutes at a time. Dolphins can drown if as little as 1 tablespoon of water gets into their lungs, so they need to be alert at all time to make sure that they only breathe in once they are above the surface of the water. The solution is to rest just one half of their brain at a time. This type of sleep is known as unihemispheric sleep and allows them to get all of the rest that they need but without losing complete concentration and preventing them from drowning. One side of the brain can be in a state of deep sleep whilst the other side is completely awake. The dolphin keeps one eye open, on the opposite side of the brain. It was first believed that dolphins evolved to be able to sleep in this way simply because of the way in which they breathe, but it is also an extremely efficient way to protect themselves against predators and make sure that they stay united with the other members of their pods.
This way of sleeping is also very useful when it comes to protecting young dolphins. Dolphin calves are very vulnerable when it comes to predators as they make handy-sized meals for killer whales and sharks. Sleeping with one eye open allows the mother to be constantly on the alert to protect their young from enemies. Even though this type of sleep is much more forgiving than falling into a complete state of unconsciousness, studies carried out on bottlenose dolphins and killer whales showed that when calves are first born, mother and baby alike barely sleep at all. They keep active 24 hours a day and if they do pause to rest they keep both eyes open. It was found that during the first few months of a calf’s life, neither mother nor baby get much sleep.
A different study, carried out on two dolphins living in captivity, found that dolphins can remain focused and on task for more than two weeks straight without pausing to sleep. From this research, scientists have concluded that dolphins probably have the ability to engage in unihemispheric sleep whilst still on the move. They think that dolphins are able to shut down one half of their brain for short periods of time without it having much effect on their concentration or focus on a task at hand.
Do dolphins sleep?