Do sharks actually sleep?

Biologists do not define sleep by pajamas, eyelids or lying down. They look for a reversible period of reduced responsiveness, a characteristic posture or inactivity, and homeostatic regulation—the tendency to compensate after sleep loss. For animals whose nervous systems differ from ours, those behavioral criteria are paired with physiological measurements whenever possible.

In a 2022 study, sleeping draughtsboard sharks had lower metabolic rates and flatter body posture than alert resting sharks. That distinction matters: it shows that sleep was not simply any period without swimming. Eye closure was a poor stand-alone indicator, especially during darkness, so the work also warns against diagnosing shark sleep from a photograph.

Researchers later recorded an electrophysiological pattern associated with behaviorally defined sleep in the same species. It is an important step toward understanding shark sleep at the neural level, but it does not make one small benthic shark a perfect model for every fast-swimming pelagic species. Sleep can evolve different outward forms while serving conserved functions.

For ram-ventilating sharks, continuous movement and sleep are not automatically incompatible. Other animals can reduce responsiveness while maintaining repetitive movement, and different brain regions need not behave identically. Direct evidence remains limited, so claims that a cruising white shark is asleep should be treated as a hypothesis rather than an established fact.