There’s a common misconception that all sharks have to keep moving to breathe. But many shark species are often seen resting in the corals, waiting for their turn to hunt.
Most shark species breathe through a system of buccal pumping, which involves pushing oxygenated water across their gills with their mouths, allowing them to breathe while at rest.
Others use a combination of buccal pumping and ram ventilation – swimming through the water to breathe – to make sure they get enough oxygen. Again, these sharks can still breathe when they stop moving but may position themselves facing oncoming water currents while at rest to make it easier.
Other sharks have spiracles, which can help the shark take in oxygen while it’s feeding or resting. These are all means by which a shark might take a break from swimming and not suffocate. And all these sharks would have a lot less trouble falling asleep because of them.
Great whites, however, are known as obligate ram ventilators. This means they can only breathe through ram ventilation, and they must keep moving in order to do so. This leads to the question, do great whites sleep?
We don’t know if great whites sleep. In order to keep breathing at rest, they may engage in a system of deep rest periods, during which they use ocean currents to pass water over their gills while shutting down parts of the brain at a time and keeping activity to a minimum.
Sleep is seemingly necessary for almost every organism on earth, so this likely applies to sharks too, and from what we can gather, there are some shark species that sleep.
So surely, an animal like the great white needs to keep moving while at the same time resting its brain.
But how exactly does this work? This is still a matter of debate, and like so many things with sharks – especially great whites – still very much of a mystery. Let’s take a look at what we do know.
Do all Organisms Sleep?
Sleep in terrestrial animals is well documented and ubiquitous. Even plants demonstrate periods of unresponsiveness during their cycle and can suffer and die without rest from daylight. Sleep in the ocean, however, is less understood.
The ocean, at a glance, seems no different in the need for its inhabitants to shut down and recover. In fact, it’s from studying animals in the ocean that we realize just how necessary sleep is to life itself.
Sleep has been described in organisms from very different lineages.
Octopuses, which are from the mollusk phylum (the same branch as slugs and snails), have been shown to exhibit what looks like REM sleep stages. This is significant because it means that sleep has evolved independently in both vertebrates and cephalopods – and, incidentally, suggests that octopuses are capable of dreaming!
This also confirms the importance of sleep across the animal kingdom, even in the ocean. It has been documented in nematodes, zebrafish, and even jellyfish.
There is seemingly no animal alive – perhaps no organism – that doesn’t exhibit some form of sleep or another when we look closely. This goes to show just how significant it is to the perpetuation of life itself.
The need for sleep, on one hand, seems obvious. In vertebrates, rest periods allow central nervous system fluids to clear out plaque build-up in the brain, hormonal shifts allow cells to regenerate, and various learning and predictive mental processes are accelerated.
On the other, sleep seems like a very strange adaptation; dropping all defenses for a third of the day seems particularly dangerous, and missing out on feeding and mating opportunities seems like it would put an organism at a disadvantage. This is especially true of animals like the great white, which would suffocate and die if they sank to the bottom of the ocean.
But it seems like these are all evolutionary sacrifices for the greater good, no matter what species you are. In this case, it stands to reason that a great white should sleep. But to breathe, they have to keep swimming, so how do they do it?
How do Great White Sharks Sleep?
Sleep data in sharks is still extremely limited. There are recorded periods of inactivity that resemble sleep in some sharks, but these are in two buccal-pumping species, and even this is inconclusive.
When kept from these rest periods, these sharks show no signs of needing to ‘catch up’ on rest. So are they really sleeping?
Great whites also show periods of relative rest. While they have to keep moving to breathe, movement is relative, and there’s no technical difference between a shark swimming through still water and a shark maintaining its position in flowing water when it comes to the laminar flow of water across its gills.
Therefore, in theory, a great white could position itself facing an oncoming ocean current and, with the minimal controlling motion of its tail, maintain that position indefinitely while resting or even possibly sleeping.
This idea has been supported with video evidence from 2016, in which a female great white was filmed slowly swimming up an ocean current during the nighttime, seemingly in a state of slumber. Still, this is very weak evidence for a condition that is exceptionally hard to measure, and as such, the jury remains out on the topic.
This addresses part of the problem, namely, how to breathe without moving. But the shark still has to activate its tail and stabilize itself against the current to some extent, which means we’re not much closer to knowing how it does that while asleep.
Other marine animals, such as dolphins, engage in unihemispheric sleep. Great white sharks may have a similar adaptation to rest, allowing them to shut down parts of their brain at a time while they continue to swim against the current, but this is speculation.
But more importantly, it’s been shown in other species that it may not be the brain that’s responsible for all the tail work back there. So are they swimming while asleep or sleeping a bit while they swim? Let’s dive a little deeper.
Do Sharks Swim while they Sleep?
So, we have established that some species of shark show signs of sleep. Draughtsboard sharks’ metabolic rate drops, and their body position relaxes during rest periods, and so far, this is the closest we have come to witnessing a shark ‘sleep’. But these are buccal pumpers and don’t have to swim while sleeping, so what about the obligate ram ventilators?
As mentioned, defining sleep can be tricky in sharks, but let’s assume that the ‘slumber state’ witnessed in the great white counts. Then it can be said that yes, some sharks have the ability to swim while they’re asleep. At least while part of their brain is at rest.
The mechanism behind how a shark swims is possibly best understood by looking at a shark’s brain and nervous system. The white shark has a Y-shaped brain that takes up only 0.008% of its mass. However, it’s very compact, containing many supporting structures and nodules.
The brain and spinal column control different parts of the shark’s physiology, and while its reaction to stimuli seems to come from the brain, the coordination of its swimming muscles may not.
Experiments involving severing the spinal cord of smaller sharks have shown that even without direct control of the brain, swimming can continue for hours. This suggests that it’s possible to shut off higher functions of the great white brain and continue to move through the water since it isn’t only the brain responsible for the motion of the swimming muscles.
It’s uncertain whether great white sharks sleep. They’ve certainly been seen to behave like they’re resting.
And brand-new research suggests that some other types of shark might be sleeping, maybe…
Sleep is such a seemingly important and ubiquitous element of life that it does seem likely every organism engages in it in some form or another. This leads to the implication that they must sleep.
The evidence available so far suggests that with a strategy of positioning itself facing an oncoming ocean current, a great white can limit its motion to the bare minimum.
It’s also known that some species of sharks can swim without brain input to the tail. If we combine these facts, assuming great whites can also power themselves without the brain, it’s possible to imagine that great whites can engage in sleep of some form. Ultimately, however, this branch of research is in its infancy. More information definitely needs to come in, but with difficulties in defining, measuring, and recording sleep in sharks, it will be a long time before there is a solid answer.