How do sharks sleep? Do sharks sleep with their eyes open?
Contrary to popular belief, sharks do ‘sleep’, that is, if we consider deep rest while remaining motionless and observing the environment the same as sleeping.
But some sharks have to keep on moving in order to breathe, one would say!
Therefore, to answer how sharks sleep, we must first see how they breathe.
How Do Sharks Breathe?
So when do sharks sleep if they have to keep swimming to breathe?
This statement is partially true as only a couple of dozen out of 500 shark species must constantly swim so they can move water into their mouths and out of their gills.
Known as obligate ram ventilators, this group of sharks includes some of the most known breeds out there, including the great white, hammerhead, and whale sharks.
However, some great whites have even been spotted using their fins to hover around 200 meters against strong currents, where they effortlessly take in water while resting.
In fact, most species of sharks, including lemon and nurse sharks, can rest on the seabed and breathe with a buccal pumping system that allows them to gulp oxygen-rich water, a unique trait of shark evolution that has existed long before even the planet’s first trees.
Some still, like angel sharks and sawsharks, even have their mouths buried in the sand and use spiracles—openings behind their eyes that function as snorkels for the gill system.
How Do Sharks Sleep?
Now that we know how they breathe, we can answer when and where do sharks sleep.
While the shark sleeping process of most species involves them resting on the ocean floor and lowering their metabolism and oxygen consumption, those that need to keep moving with an open mouth most likely alternate between active and sleep swimming.
To avoid using energy, some scientists even suggest they practice a yo-yo swimming technique that entails surfacing then descending with the help of gravity and resting along the way, provided they cannot rest while standing against currents like the great white.
In any case, these obligate ram ventilators show reduced brain activity during a restful period, as their spinal cord is most likely in charge of swimming instead of the brain.
Do sharks sleep at night?
Sharks do not follow a sleep pattern with long periods of rest and active states like mammals, but their sleeping behavior may still be determined by the day and night cycle.
For instance, some species of sharks are especially fast nocturnal hunters, so they rest more during the light of day as they expand more energy during the night.
Still, there are shark groups without any sleeping preference in terms of day or night, and they only rely on their instincts to determine whether to relax or not.
Do sharks sleep with their eyes open?
Since most sharks rest instead of completely losing consciousness, they do not close their eyes as they prefer to remain vigilant for passing prey or predator attacks. However, even those that close their eyelids do it partially due to factors such as the presence of light.
How long do sharks sleep?
Scientists cannot precisely predict the length of sharks’ resting periods as they vary between different species, how оften they repeat their sleeping cycle, and any number of external factors such as changes in lighting conditions and the movement of other sea life.
All in all, most shark species go through wakeful and deep rest periods throughout the day. Even though they do not sleep the same way humans do, they know how to take a break now and then and relax on the seabed or while gliding down slowly from the surface.
In any case, if you see a particularly sluggish shark in the wild, keep your distance to avoid becoming one more shark attack statistic, as they follow everything around them.
Can sharks sleep?
While sharks seem like they are sleeping, most of them cycle between active and deep rest states, during which they remain alert to the environment to a certain degree.
When do sharks sleep?
Most shark species prefer resting during the day as they hunt at night.
Where do sharks sleep?
Depending on how do sharks sleep and their breathing mechanism, they can sleep on the ocean floor, suspend themselves against sea currents, glide from the surface to the floor, or even gather in herds and sleep in sea caves.