Do Sharks Sleep?
Can A Shark Stop, Close its Eyes and Rest?
Although at one time it was commonly thought that all sharks were constantly on the move, it is now well known that many species of sharks will often take a break from constant swimming. However, less understood is a shark’s ability to rest and how shark sleep compares to human sleep.
Signs of Sleep in Sharks and Humans
Humans can easily tell when a friend is falling asleep because of the way he stops moving and closes his eyes, usually at night. Scientists can further study human sleep using an electroencephalogram (EEG) to record brain activity or an electroculogram (EOG) to watch eye movements. In sharks studying sleep is much more difficult. Sharks do not display habits that humans can quickly recognize as sleep and even among sharks different species show different lifestyles and behaviours.
Do Sharks Stop and Close Their Eyelids?
Some species of shark need to constantly move in order to breathe, such as hammerheads and great whites. Other species of shark do stop and sit on the ocean bottom. However it is questionable whether even these motionless sharks are sleeping. If sharks do sleep they don’t close their eyes to do it. According to the San Diego Natural History Museum’s page “Shark School > FAQs”, sharks can’t even blink. Although sharks do have upper and lower eyelids, these don’t close. (Although sometimes a nictitating membrane will move over the eyeball to protect it.) Shark expert and zoologist, R. Aidin Martin at the ReefQuest Center for Shark Research says in “How Do Sharks Swim When Asleep” that these motionless sharks can follow divers with their eyes – suggesting they are not in fact asleep.
Sharks and the Circadian Rhythm
The circadian rhythm is sometimes referred to as the ‘internal clock’ and it regulates a creature’s biology throughout a 24-hour period. For example, a person generally sleeps at night and is active during the day. Studies show that, some sharks at least, do have a circadian rhythm. Larry Glenn Allen et al describe in the book Ecology of marine fishes: California and adjacent waters how the swell shark and horn shark have a nocturnal rhythm. These sharks are active at night and during the day they find rocks to hole up in.
The Shark Brain and Sleep
Sharks that are obligate ram ventilators can never stop moving. However, R. Aidan Martin describes an experiment that may explain how these sharks manage to continuously swim without rest. Studies done on the Spiny Dogfish shark (Squalus acanthius) showed that sharks co-ordinate their swimming movements through a ‘Central Pattern Generator’ in their spinal chord. Since this generator is in the spine and not the brain the shark can continue to swim even while unconscious. R. Aidan Martin also suggests the possibility that parts of the shark’s brain shut down while others continue to run the body – a shark version of sleep walking.
Learn more about marine creatures and sleep: Dolphins Sleep with Half Their Brain
Shark Sleep is a Mystery
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