Do sharks sleep (and how?)

For millions of years sharks swam seas, from the shallowest to the deepest imaginable depths. There are more than 500 species of sharks alive today. However, despite extensive studies, there’s still a lot we don’t know about them. One of the most interesting questions asked about sharks is whether they sleep or not, and it’s one that has often sparked a lot of debate. It’s pretty obvious that sharks don’t catch forty winks in the traditional sense, which involves hours of deep, restful sleep. So, when they sleep, how do they do it? Join us as we discover if sharks really sleep and how they do it.

Can sharks stop swimming?

The biggest hurdle for sharks to take a nap is easily a minor problem with breathing. Many sharks can never stop swimming because they have to be in constant motion to breathe. Swimming retains water move over their gills so they can survive. Basically, if they stop moving, they can no longer get oxygen from the water and they die. Sharks that must swim to breathe are known as “mandatory ram ventilators” because of the way they must swim quickly through the sea with their mouths open to force water over their gills.

However, there is an exception to the rule. Some sharks have spiracles that do the work for them. Spiracles are openings behind each eye. These openings are used to draw water into the oral cavity before it is pushed back out through the gills. Spiracles allow sharks to breathe without having to swim and are essential for bottom sharks. This is because bottom sharks often wait for their prey before lunging at them. Spiracles are found on most sharks apart from requiem sharks, shortfin mako sharks (such as the great white) and hammerhead sharks – all of which have to swim constantly. So sharks like nurse sharks and sand tiger sharks both of which have a siphon can stop swimming.

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Do sharks sleep?

The question of whether sharks sleep or not has long been a matter of debate, mainly because it is quite difficult to define what “sleep” is for a shark. However, as we just discussed, sharks fall into two categories: sharks that can stop swimming and sharks that cannot (requiem and hammerhead sharks). Taking this into consideration, “sleep” will be different for these two categories of sharks.

While those who can’t stop swimming are unlikely to fall into a deep sleep (in case they accidentally stop swimming), they seem to go through periods of being active and then less active. This indicates that they can rest part of their brain while still being conscious enough to swim.

On the other hand, it should be easier for sharks with spiracles to sleep because they can stop swimming. However, it is not quite that simple. They can’t just stop swimming to nap wherever they want because when sharks stop swimming, they sink. This is because they do not have a swim bladder. Although their large oil-filled livers provide some buoyancy, they cannot just float around. Instead, sharks rely on the movement of their pectoral and caudal fins to keep them afloat. Therefore, sharks that stop swimming to rest must do so at the bottom of the ocean. However, when awake and moving, they generally breathe in the same way that obligate ram fan sharks do: swimming with their mouths open and pushing the water over their gills through movement.

Sharks that live in the middle or top of the water column rarely have spiracles. This is because the seabed or coral reefs (where they should rest) are much deeper than their natural habitat.

How do sharks sleep?

So we’ve found that sharks do sleep, although it’s more of a quiet period than a really deep sleep. Sharks with spiracles can rest on the seabed to sleep while their spiracles continue to push water over their gills. This means they can rest without worrying about swimming to breathe. However, studies have shown that even while they are asleep they are still somewhat aware of prey and potential threats entering their environment. They even sleep with their eyes open!

However, as we mentioned above, sharks love requiem sharks and hammerhead sharks only enter into quiet or less active periods to sleep because they need to keep moving. For a long time, this was thought to be “sleep swimming,” where they rest only part of their brains at a time. However, studies have shown that swimming doesn’t actually work in the brain at all. They even believe that it is actually the spinal cord that makes sharks swim. This means that those who have to swim to survive can actually enjoy some pretty restful sleep while their bodies continue to swim anyway. This soothing swim is usually done face-to-face with the current so that the water enters their mouths with little effort.

Another way sharks sleep is through a method known as “yo-yo swimming.” This method is used when sharks swim long distances. Sharks swim to the surface and then “slide” back down the water column without much swimming effort. This allows them to rest during the descent where they don’t put in as much effort. Yo-yo swimming has been observed in great white sharks migrating between Australia and South Africa.

When do sharks sleep?

Another question that often comes up when discussing how sharks sleep is when they actually sleep. Do they sleep like they are tired? Or is their sleep pattern determined by day and night like ours? The truth is that sharks’ sleep actually depends on their Circadian Rhythm.

Rather than sleeping during the day and being awake at night (nocturnal) or vice versa, most (but not all) sharks do not have a strict preference for day or night. Instead, they sleep when their instincts tell them to. However, since we’ve already learned that sharks’ sleep are simply periods of rest compared to active periods, they actually go through many of these periods in a twenty-four hour period. Therefore, it is actually very difficult to know how often sharks sleep and how much rest they get when they do.