You may have heard that sharks need to keep swimming in order to stay alive, so how in the hell are they supposed to sleep then if they need to keep swimming at all times? Well, let’s find out…
Do sharks sleep at night? Sharks do indeed sleep at night though it might not be the type of sleep humans are used to. They merely rest (snooze) because they need to keep moving.
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Why do sharks need to keep moving?
All sharks need to keep water flowing over their gills in order for them to breathe, but it is not true that all sharks need to keep moving to do so. Some are able to breathe while not swimming and do so by using their spiracles to force water over their gills.
Spiracles are openings just behind the eyes of all rays and some sharks. They help a shark to breathe when it is lying on the sea bottom or even buried right under the sand.
Sharks are not truly asleep as their eyes remain open while in state of “sleep” and even actively follow the movements of divers swimming around them.
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The sharks that need to swim to breathe, go through a process known as sleep swimming. In this state the shark is essentially unconscious.
In some sharks, like the spiny dogfish shark, the spinal cord coordinates the swimming and not the brain and that’s why they can continue to swim while sleeping.
Scientists believe this may also be the case in larger shark species as in 2016 a great white shark was captured on video for the first time in a state researchers believed was sleep swimming (video above).
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Let’s cover some of the related questions about the sharks sleeping.
Do sharks swim with their eyes open?
Do sharks sleep with their eyes open? Yes, they do. They can even actively follow the movements of divers swimming around them.
Do sharks have eyelids? Do sharks blink?
Contrary to popular belief, sharks do have eyelids but they don’t need to blink because the surrounding water cleans their eyes. Isn’t that sweet? Some sharks have a membrane that covers the eyes while hunting and when the shark is being attacked. That membrane is called the nictitating membrane.
The great white shark, does not have this membrane, but instead, they roll their eyes backwards to protect them when striking prey.
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Do sharks sleep upside down?
Sharks do not sleep upside down but they can go into a state called tonic immobility. It is a state of paralysis in which the sharks stay for an average of fifteen minutes before it recovers. Some sharks go into tonic immobility when they are turned upside down. This can be induced by placing hands lightly on the sides of the animal’s snout approximate to the general area surrounding its eyes.
A female orca was seen holding the shark upside down to induce tonic immobility off the coast of California. It kept the shark still for fifteen minutes, causing it to suffocate to death.
Do sharks get tired of swimming?
Not all sharks have to keep moving to stay alive. The ones that don’t have to keep moving can stay still on the bottom of the sea and rest. The other species of sharks, that have to keep swimming, will swim at a slight upward angle for a period of time and then rest as they slowly glide down and forward. They zig-zag up and down through the water column.
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Do sharks hunt at night?
The importance of sight in shark hunting behavior is debated. Sharks have good night vision while being colorblind. Sharks will hunt day or night. It does not matter to them what time of day it is.