It is a common misconception that sharks as a species never sleep because they must keep moving to stay alive. Sharks do engage in periods of rest throughout the day, but it is much different from the kind of sleep that other animals engage in.
It is true that many types of sharks must keep moving in order to receive life-giving oxygen from the water passing through their gills. These kinds of sharks are known as obligate ram ventilators because they draw water in through their mouths and force it out through their gills. Many sharks use a method called buccal pumping, in which water is pulled in through the mouth and forced out through the gills by the cheek muscles. Other types of sharks are able to remain stationary because they possess special structures called spiracles, which force water through their gills. Some sharks use both spiracles and buccal pumping. If any of these species were to stop swimming because, for example, they were caught in a net, they would ultimately suffocate.
Whatever method they use to breathe, sharks are able to engage in periods of deep rest while still but do not fall asleep in the traditional sense. Their eyes remain perpetually open, and their pupils still monitor the motion of creatures swimming around them. Sharks that are able to rest while stationary include the whitetip reef shark, the Caribbean reef shark, the nurse shark, the wobbegong, and the lemon shark.
The great white shark has long been a subject of interest among marine biologists because so little is known of its life processes. One big question is, of course, “Do great white sharks sleep?” In 2016, researchers studying great white sharks off Guadalupe Island, near the coast of Mexico’s Baja California Peninsula, learned the answer when they came across a female that appeared to be in a state of slumber. Following her with a robotic submersible, they watched for several minutes as she drifted in shallow water against a strong current, her mouth open so the water passed through her gills, in what appeared to be an almost catatonic state.