Sharks need to keep water moving over their gills so that they receive oxygen. It was thought for a long time that sharks needed to move constantly in order to survive. This might mean that sharks couldn’t stop, and therefore couldn’t sleep. Is this true?
Despite all the research on sharks over the years, shark sleep still seems to be a bit of a mystery. Explore the latest thoughts on whether sharks sleep.
True or False: a Shark Will Die If It Stops Moving
Well, it’s sort of true. But also false. There are over 400 species of sharks. Some need to move pretty much all the time to keep water moving over their gills so that they can breathe. Some sharks have structures called spiracles which allow them to breathe while they are lying on the ocean bottom. A spiracle is a small opening behind each eye. This structure forces water across the shark’s gills so the shark can be still when it rests. This structure is handy for bottom-dwelling shark relatives like rays and skates, and sharks like wobbegong sharks, who ambush their prey by launching themselves off the ocean bottom when a fish passes by.
So Do Sharks Sleep?
Well, the question of how sharks sleep depends on how you define sleep. According to the Merriam-Webster online dictionary, sleep is “the natural periodic suspension of consciousness during which the powers of the body are restored.” We’re not sure sharks are able to suspend their consciousness, although it may be possible. Do sharks curl up and rest for several hours at a time, like humans generally do? That’s not likely.
Shark species that need to swim constantly to keep water moving over their gills seem to have active periods and restful periods, rather than undergoing deep sleep as we do. They seem to be “sleep swimming,” with parts of their brain being less active, or “resting,” while the shark remains swimming.
At least one study has indicated that the shark’s spinal cord, rather than the brain, coordinates swimming movements. This would make it possible for sharks to swim while they are essentially unconscious (fulfilling the suspending consciousness part of the dictionary definition), thus also resting their brain.
Resting on the Bottom
Sharks such as Caribbean reef sharks, nurse sharks, and lemon sharks have been seen lying on the ocean bottom and in caves, but they seem to continue to watch what is going on around them during this time, so it’s not definite that they are sleeping.
Florida Program for Shark Research Director George H. Burgess discussed the lack of knowledge around shark sleep with the Van Winkle’s blog and says some sharks may rest during “yo-yo swimming,” when they actively swim to the surface but rest as they descend. Whether they actually rest or dream, and how resting varies among species, we don’t really know.
However they actually get their rest, sharks, like other marine animals, don’t seem to fall into a deep sleep as we do.
Florida Museum of Natural History Department of Ichthyology. Shark
Grossman, J. 2015. How Do Sharks Sleep? Do They Dream? Van Winkle’s.
Martin, R.A. How Do Sharks Swim When Asleep? ReefQuest Centre for Shark Research.
Martin, R.A. 40 Winks Under the Sea. ReefQuest Centre for Shark Research.