Med thumb shark

If you’re asking whether sharks snuggle into a crevice in a coral reef for their #hardeight, the answer is, no, they don’t. But if you define sleep as “resting, not eating, and conserving energy,” as George H. Burgess, director of the Florida Program for Shark Research at the Florida Museum of Natural History, does, then yes, sharks do sleep. Meaning — they have both active and restful periods. Just like us, only different.

Sharks get their oxygen by pumping water through their gills. But while most fish use muscles to pump that water, sharks have a different type of ventilation system, requiring movement to extract the oxygen, and to breathe. It’s logical, then, for people to assume that since a shark has to swim to stay alive, it can never sleep.

To solve this problem, the big, toothy oceanic breeds — what most people picture when they think of a shark — do something called “yo-yo diving.” They swim up toward the water’s surface, enter a restful state, then lazily descend, continuing to zig-zag for several minutes at a time. Interestingly, according to Burgess, sharks typically yo-yo dive at night.

Other shark species do it a bit differently. Those that spend most of their time on the ocean floor, like nurse sharks, dogfish sharks and catfish sharks, can force water over their gills by opening and closing their mouths.

This allows the bottom sharks to rest. Though, again, whether or not this activity is “sleep” as we understand it is debatable.

Sharks, for instance, do not have eyelids, and it’s totally unknown whether or not they engage in REM, the rapid eye movement phase of sleep in which we dream. REM and non-REM are the two phases that pretty much define sleep as we know it.

Really, the whole thing is still a mystery, admits Burgess. But, for his money, the shark’s sleeping behavior meets the necessary requirements.