Most species of sharks sleep very little.
The jaw muscles of sharks are not capable of pumping water through their gills on their own. These predators are forced to move in order to capture water with their mouths. Therefore, a shark can remain in a state of rest for no more than an hour. If it sleeps for too long, it risks dying from a lack of oxygen.
Different species of sharks solve this problem in various ways. For example, in dogfish sharks and spiny dogfish, the spinal cord controls the swimming muscles, which may allow them to move while sleeping, like sleepwalkers.
Some species of sharks, such as nurse sharks from the family of hammerhead sharks, have been observed lying on the bottom for extended periods. They do not swim but continue to breathe by opening and closing their mouths, synchronizing these movements with the action of their throat and gill muscles. Additionally, many benthic species have openings behind their eyes (spiracles) that aid in water circulation: through the spiracles, the shark draws in water, which is directed to the gills through a special channel.
It is also possible that some sharks are capable of sleeping "halfway," like dolphins, where the hemispheres of their brains rest alternately. Such a shark can doze while simultaneously controlling its fins and tail, allowing it to move and, consequently, breathe.
