Here’s a question that many shark fans ask themselves… How do sharks sleep? Well, they don’t sleep, exactly.
Sharks do not experience sleep the same way humans do. Some can’t sleep at all, and the ones that do never close their eyes. Some shark species do, however, cycle through alternating periods of alert wakefulness and profound rest that is similar to sleep. We are pretty certain that sharks do not dream the way humans and some other animals do.
Like all fish, sharks breathe through respiratory organs called gills. One thing that all sharks have in common is the physiologic need for a constant flow of water to move through their gills. Some sharks achieve this by remaining in motion at all times. Moving sharks may not sleep at all. Sharks that can stop swimming in order to rest use specialized apparatus known as spiracles to force oxygen-rich water through their gill system. Rays and skates, which are close relatives of sharks, also use spiracles to breathe.
What types of sharks sleep?
Many sharks that inhabit the pelagic regions of the open seas, relatively far from shore and nowhere near the bottom, swim all the time and do not sleep. If they are restrained in something like a fishing net (which happens all too frequently!), they experience hypoxia, or lack of oxygen, and drown. Marine biologists refer to sharks with this survival imperative to swim as obligate ram ventilators. Ram in this case describes the way sharks ‘inhale’ through open mouth and ‘ram’ water backwards through their gills.
Sharks that must move in order to breathe may also be called buccal pumpers, referring to the buccinator, or cheek muscles they use to move water inward and through their gills. Ram ventilators and buccal pumping sharks can increase their oxygen levels and energy by swimming faster with their mouths open wide.
Obligate ram ventilating sharks include:
Recent studies show that it is the spinal cord, not the brain, that causes sharks to swim. For this reason, it is now believed that some always-moving sharks may experience rest periods wherein their brains are less active.
Generally, sharks that dwell at the bottom of deep and shallow seas are able to stop moving and remain at rest, or near-sleep, on a coral reef or sandy sea bottom. While wide awake and in motion, bottom dwelling sharks ventilate the same way as pelagic sharks. When they require deep rest, they sink or swim to the bottom where they sleep while breathing via the spiracles behind their eyes.
Sharks that ‘sleep’ while immobile include:
The general consensus among marine biologists is that most sharks experience cycles of awareness and less-conscious states.
How do sharks protect themselves while sleeping?
Evidence indicates that sharks, like dolphins (which are mammals, not fish) may “turn off” one side of their brain when they go into a deep resting cycle. Forget about sleeping with one eye open. When a shark is in a deep rest period, half its brain is active, and both of its eyes are always open!
Sharks don’t close their eyes, because they have no eyelids. Instead, they have a translucent “nictating membrane” that covers the eyeball right before the shark bites its prey.
So, what do sharks do when they’re not sleeping?
Most sharks spend their time cruising calmly and eating. Almost all sharks are carnivores, and most of them munch on fish. It is worth noting that the biggest shark of all, the massive forty-foot long Whale shark, consumes plankton, which are some of the smallest creatures in the ocean.
Sharks rarely attack humans. When a shark attack does occur, it is usually a case of mistaken identity. Surfers suffer more shark bites than anyone, generally because they looked like a savory seal or tasty sea turtle to a nearby shark.
Sharks are remarkable creatures, perfectly suited to survival. Carbon dating of excavated shark teeth proves that sharks, in forms very similar to 21st century sharks, swam the seas of Earth long before the appearance of dinosaurs. Just imagine that. Sharks have been able to survive mass extinctions of the most powerful creatures on Earth, and yet they have still survived. Interestingly enough, from what we understand, the biggest threat to sharks since the beginning of their existence is HUMANS!
Awake or asleep, sharks are a vital part of the ecosystem
Despite their dodgy reputation as savage man-eating killers, most sharks cause no harm to humans. In fact, people are far more dangerous to shark populations than any ocean dwelling shark species is to people. Sharks may kill around a dozen people annually, but humans kill tens of millions of sharks every year on the low end, and hundreds of millions on the high end. By the time you are finished reading this article, many more sharks will have died at the hands of humans. So who is the REAL mass killing predator?
If all the sharks around the globe disappeared tomorrow, the oceans would not survive for long. Sharks are a crucial part of the food chain and work hard to protect healthy ocean ecology. The more you understand about these amazing sea creatures, the less you are likely to fear them, and the more likely you are to appreciate the massively important role they play in our own survival.