By Blaise Jones
The exact nature and biological reason behind sleep is still a mystery to science. No one knows for certain why any animals sleeps, and for that matter what sleep even really is. Therefore, it is very difficult to determine whether sharks actually sleep or not. However, there is evidence that many species of shark do engage in behaviors similar to sleep.
What exactly is Sleep?
While the exact nature of sleep is still unknown, scientists identify the behavior in vertebrate animals as, “a reduction of activity and metabolism, and a decreased sensitivity to external stimuli.” While there are species of sharks that display aspects of this behavior, at the same time they do not display all of it.
The Worst Case of Sleep Apnea Ever
As a group, sharks need water to pass over their gills in order to breathe. Most sharks cannot actively pump water over their gills like bony fishes can, and thus have to keep moving in order to breathe. So if a shark stopped to take a nap, it would stop breathing.
However, there are species of shark that are able to rest on the bottom. Species such as the nurse and the wobbegong shark have spiracles, which allows them to pump water over their gills without moving. The whitetip reef shark uses another method to remain on the bottom, known as buccal pumping.
Buccal pumping is when the shark is able to suck water in through its mouth and over its gills. But instead of using the spiracle, these sharks have developed powerful specialized cheek muscles that allow them to efficiently draw in water through their mouths and over their gills.
In fact, shark species with spiracles use buccal pumping as well. The key difference is the method. Sharks without spiracles rely upon their cheek muscles to suck in the water, whereas sharks with spiracles suck the water in through the spiracle tube.
Down but not Out
During the day these species can be found resting on the bottom in secluded locations, and appear to be sleeping. However, while these sharks do show lowered activity, they are not fully asleep. These resting sharks still react to external stimuli. The sharks follow nearby animals with their eyes, and if approached too closely they will get up off the bottom and swim away.
Using (Part of) Your Head
As for the species of sharks that cannot stop swimming, there are theories on how they “sleep”. While there is no hard evidence for this, some scientists theorize that sharks are able to shut down parts of their brain sequentially, instead of all at once. This is how dolphins are able to sleep.
By doing this it would allow sharks to basically turn off all nonessential functions of their body while still being able to swim enough to keep from drowning. After a while the shark could switch over their motor controls to the deactivated part of their brain, reactivating it and allowing the half that had just been running to shut down and enter “sleep mode.”
“The Encyclopedia of Sharks” by Steve Parker
Sharks: The Animal Answer Guide” by Gene Helfman and George H. Burgess
Alan Moore, Director of the Coastal Marine Education and Research Academy