Fish are asleep, but not all. Moreover, they do not sleep in the literal sense of the word. The fact is that they will not be able to rest like people because of the physiological structure of the eye and the absence of a swim bladder in some species of fish.
How do fish sleep?
Any person needs to close their eyelids in order to fall asleep. Fish have no eyelids that they could close, falling asleep in a sound and healthy sleep. However, this does not prevent them from resting, which is called "fishy". Some of them generally lie on their side. In fact, they don't close their eyes.
This is not to say that fish are unconscious, like a person during sleep. Yes, their consciousness is dulled, but not so much. Physical functions are suspended for a while, and the fish itself may not pay any attention to what is happening around. However, with all this, the brain receptors are on alert, which allows her to instantly get out of her stupor.
There is a great variety of fish species on Earth. Many of them have specific periods of activity and rest. The swim bladder helps them to "sleep" while staying afloat. It is worth noting that not all fish have this organ, so some of them generally have to sleep in motion. These include all bottom and deep-sea fish species. Their "sleep" is a short respite during their continuous movement. That is why such rest can be called “sleep” conditionally.
No sleep and no rest
One of the brightest representatives of benthic fish that do not have a swim bladder are, of course, sharks. In order not to drown during sleep, these predators need to actively swim. Unlike humans and many land animals, for whom swimming is a rather short-term occupation, sharks are forced to involuntarily be in constant motion from the first day of their life to the last!
The absence of a swim bladder does not allow sharks to always remain "suspended" in the depths of the depths, as many other species of fish can do. If the shark stops its wave-like movements with its muscular fins and protruding tail even for a moment, then the gravity of its own body will inexorably pull it to the bottom! It is curious that this is why dead sharks never float to the surface, but fall like a stone to the bottom.
As mentioned above, in the traditional understanding of sleep, these fish never sleep. They are in constant motion both day and night. However, some of them still manage to rest. Special species of sharks that live in coastal waters swim into small underwater caves located at shallow depths of rivers or lakes. There they lie on rocky ledges or at the bottom.
It can be concluded that in the literal sense of this understanding, not a single fish sleeps, however, some lucky ones, whom God has awarded with swimming bladders, can still afford to rest somehow, which cannot be said about bottom and deep-sea fish.
