Do fish sleep?

Although it is not so common to observe fish sleeping, compared to other groups of animals such as birds and mammals, various scientific studies affirm that both fish do sleep , both bone and cartilaginous and agnate.

However, it is difficult to establish that fish sleep following the same criteria and behavior patterns that characterize sleep in mammals, for example. It is not common for fish to maintain a fixed posture while sleeping, to have a quick response upon awakening, or to easily count on being active again if disturbed.

In the next sections of this article we will discover exactly how fish sleep , what behavior patterns characterize them, as well as some curiosities about the rest of sharks, one of the most curious fish there is.

There are different techniques that fish use to sleep , so establishing a single sleeping method within the world of fish would be quite wrong. Let’s get to know some of these most used techniques more closely and we will better understand how fish sleep :

• Many species of fish rest on their rear or lower fin , which they place on the seabed as an anchor. In this way, the fish can sleep and ensure that they will not be swept away by possible ocean currents of great force and pressure.
• As many other species of fish choose to rest directly on the seabed (or freshwater, depending on the habitat in which they are found). In these cases they must make use of one of their most useful internal organs, the swim bladder. Thanks to it, the fish are able to rise to the surface on occasions when they need oxygen from the air to meet their respiratory needs, and return again to the seabed / freshwater without even waking up.
• Those fish that lack a swim bladder and, therefore, cannot use the prior art to surface to breathe at the same time as they sleep, must remain in constant motion , being able to sleep. This is the incredible and surprising case of sharks, which we will see in more detail in the next section.

Did you know that sharks never stop ? Not even to rest, although this does not mean that they refuse to fulfill their rest and sleep needs. So how do sharks sleep?

As we have commented previously, fish that lack a swim bladder are those that are characterized by sleeping without remaining immobile, among them, sharks. These are perhaps the best known for their habit of sleeping while in constant motion , since if this were not the case, they would run the risk of falling to the seabed and could die.

In this way, sharks are able to cover both their respiratory and resting needs thanks to the action of their gills . The gills are external organs located on both sides of the body (between the head and the swim fin), they allow sharks and other fish without a swim bladder, to capture the dissolved oxygen in the water and incorporate it into their body through the respiratory system. In this oxygen uptake it is important and vital that the animal is moving, since the simple movement of the water facilitates the correct diffusion of oxygen through the gill lamellae. Without a doubt, it is one of the most fascinating events that guarantees the survival of fish.

Learn more about this question in this other post: Do sharks sleep?

Although they lack eyelids and their eyes do not remain moist like those of other groups of animals, fish have a protective membrane that allows them to sleep and rest, even when the sun’s rays reach them.

Therefore, we can affirm that fish can sleep with light , both in the presence of natural sunlight and artificial light that they have near fish tanks, aquariums or other habitats in which humans keep them.

In addition, there are many natural habitats in which the depth at which the fish live is not very high and, therefore, they sometimes receive natural or artificial light even at night. In this way, the protective membrane of the eyes that acts as an eyelid in the fish, will prevent them from being disturbed by any type of light while they sleep and replenish energy.

Finally, we encourage you to watch this curious video about animals that sleep strangely , because fish are not the only ones that rest in different ways than ours.