Do Sharks Sleep?

Sharks are one of the most feared but least understood creatures in the sea today.  The subject of myths, legends, and movies, sharks are iconic as the great hunters of the sea.  Although they are fish, sharks have very unique biologies.  One of the most commonly asked questions about sharks is whether or not they sleep. Gills work by extracting oxygen from the surrounding water.  In order to do this, fish must constantly take in water into their gills.  This is called Ram ventilation.  While most fish have the ability to do this while sitting still, sharks do not.  Sharks evolved out their ability to take in water while sitting still.  Because of this, sharks are known as Obligate ram ventilators, meaning they must constantly swim to take in oxygen.

Unusual biology

SharkSharks have an unorthodox biology, even for fish.  Unlike traditional fish, sharks have no swim bladder.  Swim bladders provide buoyancy for fish, allowing them to float in place.  However, sharks to not have this, relying on the lift their fins provide in order to stay afloat.  If sharks stop moving, they will sink to the bottom.  Because of this, sharks are in a perpetual state of motion, constantly swimming.  Sharks rarely stop and nest on the ground.  But this isn’t only for buoyancy purposes.  While this may seem like a disadvantage at first, it actually holds many advantages for the sharks.  Because of a lack of swim bladder, sharks have the ability to dive to a variety of depths instead of being limited to just one as most fish are.  It allows sharks to be more mobile, coming and going as they please. In scientific studies of sharks, scientists noticed that sharks breathed more efficiently while swimming as opposed to resting on the ocean floor.  This keeps sharks swimming in an effort to gain more oxygen.  Counter intuitively, it is more energy efficient for sharks to continue moving than to simply rest.

Do Sharks Sleep?

This then poses the question, do sharks sleep?  For years it was hard to tell.  As sharks are constantly in motion, studying them to see if they sleep was a very difficult endeavor.  While it has never been proven definitively, small clues give us insight to the question.  Scientists studying small sharks have observed that the swimming motion of sharks is coordinated by the spinal cord, and not by the brain.  For this reason it is theorized that sharks sleep by continuing to swim with their spinal cord, and let their brain sleep. However, there are a few conflicting reports that throw a wrench into this hypothesis.  Water factors such as temperature, current, and salinity may affect sharks.  Reef sharks have been observed sleeping in caves in Mexico.  Discovery of the caves yielded dozens of sharks all dozing for the night.  Close inspection of the caves determined that the water in the caves had high oxygen levels, making it more energy efficient for the sharks to stay put than keep swimming. Great white sharks have observed the most unusual behavior recorded for shark sleep studies.  Geotagged Great White sharks have been observed in the Dyer Islands travelling to gullies at the bottom of the ocean and remaining stationary for hours.  The areas they have been observed doing this have currents passing through them, presumably bringing fresh ocean water into the sharks’ gills as they rest.  This would allow them to sleep while still getting enough oxygen. While the question hasn’t been settled for sure, it’s safe to presume that sharks to sleep to some degree.  What that looks like or if it even appears to be sleep at all is up for the verdict.