How do fish sleep?

 How do fish sleep?

We have probably all asked this question at least once in our lives. Most of us, usually during our childhood, have had fish as pets, however, although we have tried to observe them sleeping, we have not obtained the answers we expected... But of course, the word sleeping does not exactly define their resting period. It seems that fish do not sleep since, apparently, they are always with their eyes open. But the simple fact that fish do not have eyelids does not mean that they do not sleep.

Like any living being, they also need a rest time, as all animals respond to the time and stationary cycles of this planet.

They sleep in their own way, motionless at the bottom of rivers, seas or aquariums. They slow their heart rate and completely suspend any movement to achieve something like a short hibernation. They have a transparent membrane that covers their eyes, preventing the dim light of the night from disturbing their "sleep". Something like our eyelids.

Fish are always alert, so they don't fall asleep in the same way we do. Like some birds, they only rest with half of their brain and the other half is constantly alert to detect predators. After an indefinite period of time they wake up and rest with the other half. They thus reduce their state of consciousness during sleep but do not enter a state of total unconsciousness.

The swim bladder (buoyancy organ, a kind of gas-filled bag) allows them to regulate their buoyancy, to stay balanced without swimming, and to store oxygen.

However, large predators such as sharks do not have a swim bladder, so they cannot regulate their height and must be in constant motion to obtain oxygen. The shark acquires water through its mouth while swimming and this water is filtered through the gills retaining oxygen. If a shark stops, the gills practically do not pick up oxygen.

There is a species of sharks that inhabits Isla Mujeres, in Mexico, where there are underwater caves called "the caves of the sleeping sharks". These sharks manage to rest by strategically placing themselves in places in the caves where the current provides them with enough oxygen to avoid drowning.