My friend was saying that shark never sleeps. Because to keep it afloat, it has to continuously swim, so that it can't sleep.
This claim is repeated in the Ocean of Know:
They certainly need to conserve their energy because they never really sleep and most of them never stop swimming.
I know shark has to continuously swim to keep it afloat. But does that mean that sharks never sleep?
Florida Museum of Natural History:
It was once believed that all sharks had to swim constantly in order to breathe and could not sleep for more than a few minutes at a time. Oxygen-rich water flows through the gills during movement allowing the shark to breathe.
While some species of sharks do need to swim constantly, this is not true for all sharks.
Some sharks such as the nurse shark have spiracles that force water across their gills allowing for stationary rest.
Sharks do not sleep like humans do, but instead have active and restful periods.
ReefQuest Centre for Shark Research:
... when it comes to whether or not sharks sleep, we just don't know.
There have been reports from all over the globe, including Mexico, Japan, and Australia, of sharks resting motionless in caves. These have been called "sleeping sharks", but I can attest that the eyes of these quiescent sharks do follow divers moving about in the caves with them, so they're definitely not asleep.
From experiments carried out on a small shark called the Spiny Dogfish (Squalus acanthias), we know that the 'Central Pattern Generator' that co-ordinates swimming movements in sharks is not located in the brain, but in the spinal chord. Thus, it is possible for an unconscious shark to swim.
There is even a possibility that the sharks' fore, mid-, and hind- brain shut down in sequence, resulting in the shark equivalent of sleep-walking.
... we don't know whether sharks sleep or not.
Many shark species need to constantly swim in order to breathe (or respire) but this does not necessarily mean that they don't "sleep".
Many animals can shut down part of their brains for a short time, while keeping other parts "awake" or alert, and watching for predators;
Other animals merely slow their brain function for a certain amount of time each day;
Sharks may do these things as well, even though they keep swimming. It's also possible that they don't sleep at all.
The only way for us to find out for sure is to catch them and measure their brain activity all the time.
It is unclear how sharks sleep.
Some sharks can lie on the bottom while actively pumping water over their gills, but their eyes remain open and actively follow divers.
When a shark is resting, they do not use their nares, but rather their spiracles. If a shark tried to use their nares while resting on the ocean floor, they would be sucking up sand rather than water. Many scientists believe this is one of the reasons sharks have spiracles.
The spiny dogfish's spinal cord, rather than its brain, coordinates swimming, so it is possible for a spiny dogfish to continue to swim while sleeping.
It is also possible that a shark can sleep with only parts of its brain in a manner similar to dolphins.
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