According to experts, sharks have been around for more than 450 million years. There are many curious and interesting facts about sharks, such as that the whale shark is the largest fish in the ocean, that sharks replace their teeth throughout their lives, or that they have very strange sleeping habits. If you like animals or you are simply a very curious person, do not miss the next article in Thefishinfo, because below we explain how sharks sleep and we reveal everything you should know about this fish.
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How sharks sleep
The truth is that sharks do not sleep, at least not as human beings understand it. What sharks do is rest, so that only part of their brain is unconscious when they perform this action. In fact, and closely linked to this, we must mention that most species of sharks rest while moving because sharks have to be in constant movement to be able to breathe.
On the other hand, there are species of sharks such as the nurse and the catfish (remember that there are approximately 465 shark species) that can keep still and continue breathing, something that is due to their ability to absorb water by mouth pumping. But this does not mean that they can sleep soundly as humans do, because they need a certain degree of consciousness to be able to breathe. 
In turn, it should be noted that the white shark has been seen sleeping or resting in a relaxed and calm way, but without stopping to move and with its eyes open. This is because the great white shark, also called the “king of sharks” or the “great shark” has certain special characteristics:
- On the one hand, this shark has a highly developed sense of smell and is capable of detecting residues of blood and other elements several kilometers away, so you should always be vigilant.
- The same goes for movement, as the king of sharks can sense any movement in the water several meters away.
- Also, we are talking about a fish with very sharp scales that allow it to move extremely quickly through the water. It can reach up to 50 km per hour when it is time to hunt.
Why sharks swim by sleeping
Most sharks, when resting, should keep moving. But why do they take these rest periods in constant motion? The easiest explanation is that sharks do not have a swim bladder like much other fish, a flotation organ that allows them not to sink.
Besides, to be able to breathe, sharks must have their mouths open and thus absorb the necessary oxygen that passes through their whites. These gills only absorb oxygen when the shark is moving, something that also explains why these fish sleep swimming.
On the other hand, it is worth mentioning that there are shark species that have blowholes. These are openings behind each eye that cause water to move through the shark’s gills, allowing the fish in question to rest at rest and without moving on the ocean floor.
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Why sharks sleep with their eyes open
To start, you should know that there are sharks that do not even have eyelids because the seawater is responsible for washing their eyes at all times. However, some sharks have a translucent membrane (or Nictitating membrane) that allows them to keep their eyes protected when they swim at high speed, when they attack or when they are attacked.
However, we can also affirm that if sharks sleep with their eyes open, it is because, remember, they are not asleep, but a part of their brain is still conscious to continue the cycle of taking air and resting at the same time.
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