Sharks do not sleep like humans do, but instead have active and restful periods. But is this sleep?

The answer is… maybe. Sharks definitely rest. They definitely “space-out”. But, it’s not clear if this is actually “sleep”, which is a reduced state of consciousness, or simply periods of reduced physical activity.

According to the Merriam-Webster online dictionary, sleep is “the natural periodic suspension of consciousness during which the powers of the body are restored.”

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. This is possibly an adaptation that allows sharks to keep swimming while their brains rest, or possibly sleep.

In another study in two species of buccal pumping sharks (that is, sharks that are able to breathe while remaining still), small electrical pulses were applied to their surrounding water while the sharks were in their motionless “restful” state which does superficially resemble sleep. Both the Port Jackson and draughtboard sharks used in this study were shown to respond to these electrical signals while actively swimming, but stronger pulses needed to be used before they were noticed by the “restful” sharks. This reduced state of awareness is a strong indicator that these sharks actually are asleep – i.e. in a diminished state of awareness, rather than just remaining still which would not have dulled their sensitivity.

Another very recent study took a different approach – seeing if the amount of time sharks spent resting had any correlation with the amount of time that sharks spent active. In other words, do sharks “sleep in” if they have been “awake” for longer? The study found that these sharks rested for the same amount of time regardless of how long they had been active. This is a strong indicator that shark rest cycles are highly dependent on their Circadian rhythms, not on other factors like in us humans. In other words, if sharks sleep, it is because their instinct is to rest at a certain time of the day, rather than because they are feeling tired.

In 2016, a great white shark, one of the few species that actually does need to keep moving to avoid death, was observed seemingly asleep with her mouth wide open facing into the current. This footage, captured by the Discovery Channel while filming near Mexico’s Baja Peninsula, appears to show the shark in a completely catatonic state, with a fairly weak swimming behaviour that many speculate is an automatic way for it to keep getting oxygen while it rests. Similar behaviours have also been observed of great white sharks resting in gullies in False Bay.

Migratory sharks, particularly great whites use a process called yo-yo swimming to cover great distances efficiently. Basically, the shark swims actively to the surface, and then “glides” in the direction it wants to go as it sinks again. This is known to be a very efficient way of covering large distances, such as the migrations of great white sharks between South Africa and Australia, and it has been suggested that sharks may use the calmer descent “yos” to nap.

So do sharks sleep? – the answer is still “we don’t know – maybe”.