Do fish ever get tired of swimming – Do they ever sleep?

Fish are optimally adapted to life in the water. Nowhere is this more evident than in their elegant and efficient ways of moving in the water. Water has a much greater density than air, so a streamlined body is a prerequisite for a good swimming technique. The structure of the skin and the shape of the tail and fins also determine the swimming speed. Many people then ask; Do fish ever get tired of swimming?

Do fish ever get tired of sleeping? – Fish don’t sleep in the same way humans do by closing their eyes and going to sleep. They do, however, rest. This is how fish gain enough energy to swim constantly in search of food and a partner to mate with. Fish do get tired and have the same instincts as humans which is to rest. They just don’t sleep as we do, they almost power nap. 

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Fish and their swimming techniques

Fish regulate their depth in the water through their gas-filled swim bladder. These can be enlarged or reduced as needed. However, cartilaginous fish such as the shark, do not have a swim bladder. They regulate their depth in water, solely by their muscular strength; for this, they must move permanently in the water, a simple ‘standing’ in the water is not possible for them.

Sharks compensate for the absence of a swim bladder with the use of their cartilaginous skeleton and their oily liver.

In addition to the Shark, there are several other species of fish which also do not have a swim bladder; Rays, Skates, Mackerels as well as the Sea Scorpion. In the case of mackerel, they are particularly agile and can escape attackers quickly. Faster than most other sea creatures, they can change their depth extremely rapidly, whether it’s pushing down into greater depths or rising to the water’s surface.

Depending on which living conditions fish have to adapt to, they have also developed different swimming techniques. Predatory fish are usually very fast, their tail fins are built so that they give the body a strong boost forward. Other fish, especially those that live in corals, must be able to change the direction of swimming very quickly. Some coral fish can even swim backward.

Do fish sleep while swimming?

Even fish sleep, okay they rest really. As with humans, sleep takes up much of the fish’s life. Many people do not know that sea creatures also practice their sleep.

As soon as night falls, fish drive down their metabolism, reducing both their breathing and their heartbeat. This not only saves energy but also keeps the creatures alert at the same time. In the case of impending dangers, they can now become active again as fast as lightning.

However, there are also fish that lie on their side to sleep. Parrotfish even envelop their body with an odor-absorbing phlegm that can protect them from potential attackers.

This use of “pajamas” is one of the fascinating things of underwater life. Fish do not actively swim during sleep, they either float in the water or hide in their shelters.

The dream of the fish

Fish, such as those we are used to seeing in aquariums, never close their eyes and that can make us think that they do not sleep. However, this is only due to an anatomical feature, since they do not have eyelids.

All animals, from unicellular organisms to more evolved ones like humans, need to sleep. This is the moment of neuronal rest in which energy is recharged, enabling them to continue with day-to-day life. Therefore, fish are no different and therefore also need to sleep, but in a very different way than humans.

With most fish, rest consists of stopping swimming and reducing both physical and metabolic activity, staying in some corner, between the rocks or in more sheltered areas dozing.

Others sink and remain at the bottom of the sea. It is more a state of rest than a deep sleep like that of humans. But it has the same result.

Unlike humans, the dream of fish does not register the REM phase. They reduce their state of consciousness, but they never become unconscious. In this way, they are always alert and ready to escape their predators.

In addition, each species of aquatic creature has developed different methods of rest. The parrotfish, for example, is covered with a transparent cocoon created by itself which is a type of mucus that it secretes from its head, and which is believed to be used to protect itself from its predators by hiding its smell.

On the other hand, some prefer to sleep at night and in the dark and others do it during the day.

At the bottom of the sea

If we consider now the characteristics of the benthic fishes, that is to say, those that live near the bottom or in direct relation with it, we see that they can be very different from those of the pelagic ones.

In the first hundred meters of the depth of the continental shelf, there are different marine environments linked to the type of substrate and the oceanographic conditions that are present there. Other factors include waves, exposure to sunlight, marine currents, the nature of the substrate, cavities, and hollows of rocks as well as populations of algae.

The number of environmental variables that condition seabed habitats are very high, therefore, fish have to survive in a very diverse environment. This has conditioned the presence of species which have adapted to live in each of these environments.

There are fish that live among algae, in small cavities, in caves, swimming among rocks, in sandy bottoms, and thus there is a long list. One of the points with the greatest biodiversity among fish is in Raja Ampat, in Papua New Guinea, where more than 1,500 species of fish have been recorded in its coral reefs.

The species that live here have a great diversity of shapes and colors. Generally, they are slow-moving creatures and do not need to travel long distances. Bream, for example, live very close to the bottom of the sea. It has to move between stretches of rocks and stones, with narrow spaces between them. This fish feeds on small invertebrates from the bottom, so it is often seen pecking between the algae and rocks. For this, it needs to be able to make very precise movements which will allow it to approach the prey and capture it.

Often the pectoral fins acquire great importance since on many occasions they act like oars. With them they help each other to stabilize, to make small turns, to advance slowly or even to retreat.

In sand or sediment bottoms, we can find, for example, rays and flounders. Their flattened shape allows them to better camouflage themselves. Their survival is dependent on going unnoticed.

Each species has their own colorations which allow them to integrate better in the environment and helps them to go unnoticed. In many cases, these colorations can vary considerably from one sex to another, as is the case of the old ones, Sparisoma cretense (a species of Parrotfish), from the Canary Islands, and can even be indicators of the degree of sexual maturity of each fish. The female of this species wears spectacular red and yellow colors while the males are gray.

In fish, their shape and color rely on adequate integration within their environment, which facilitates their survival.

Do fish ever get tired of swimming?

Let’s answer the question, Do fish ever get tired of swimming?

The short answer is Yes, they do, Hence the reason they need to rest to regain strength. 

The creatures that live in the pelagic environment never stop swimming. It’s the same with fish. The question is then do fish ever get tired of swimming? If each component of a group of divers makes a list of the fish that it encounters during a dive, it is curious to verify that there may be notable differences between them. Surely, some have focused more on the fish that move over the rocks, others where they are in the hollows and cavities at the bottom or in between the algae. This can help us to understand the large number of microhabitats and environments that exist on the seabed, especially at shallow depth, which leads to the presence of a great diversity of fish.

The same does not happen in the pelagic environment, that is, in the column of water separated from the bottom, since it is of a much more uniform medium.

These creatures swim throughout their lives and, for this, they have a large muscle mass that acts on the tail and that allows them to be active for many hours a day.

Fish that live in the pelagic domain have many common characteristics, including a uniform coloration, blue on the back and clear on the belly, their elongated and narrow shape allows them to move with as little effort as possible, or  the body is muscular and has the caudal fin in the form of a half moon or divided into two halves, which is the most effective way to propel the fish during swimming. Usually, they are always swimming, some at considerable speed and promptly need to increase it, either to hunt or to flee. The only way to reach its prey is by being faster than it. Therefore to the question, do fish ever get tired of swimming? The answer is yes.

Have you ever considered what will happen if fish stop swimming? Will they die? Find out in our latest article here- Do fish die if they stop swimming.

Water and buoyancy

The density of water opposes resistance to what moves in it. Fish are denser than water, so they tend to sink. What prevents this from happening? There are fish that live on the seabed, so this is not a problem. But others that live on other levels need to stay afloat to move. Some species, such as the shark (cartilaginous fish), need to move constantly to stay alive. What allows its buoyancy is its large fatty liver.

While other fish, such as pike (bonefish), can remain motionless because they have an inflatable or swimming bladder, which keeps them afloat. The bladder is filled with gas and the amount of it is controlled depending on the needs of the fish at that time.

How do marine mammals sleep?

The truth is that there are some aquatic creatures that never stop swimming: This does not mean that they do not rest.

For example, some species of aquatic mammals such as whales and dolphins are able to sleep and swim at the same time. They can do this by resting one half of the brain, which allows them to continue controlling their respiratory activity since otherwise, they could die.

Being aquatic mammals they need to go to the surface to take oxygen. In this way, the hemisphere of the brain that continues to function remains conscious to lead them to the surface.

Likewise, some species of sharks never stop swimming. Their breathing mechanism consists of leaving the mouth open whilst moving through the ocean. In this way, oxygen-laden water passes through the gills and can thus reach the blood. For this reason, if they stopped moving they would die.

As you can see, fish and other aquatic mammals are very different from humans. In the aquarium, we have very interesting marine species such as moray eels, starfish, sharks, and many other creatures.

Ways to swim

In addition to buoyancy, the way fish swim is determined by the shape of their body and the fins they have. The more hydrodynamic the body of the fish, the less resistance the water offers for them move.

For example, swordfish can reach a speed greater than 60 miles per hour, thanks to their hydrodynamic design. On the other hand, there is the seahorse, which due to its low hydrodynamic shape is a very slow swimmer; although it can beat its small dorsal fin about 35 times per second, the vertical position of its body does not allow it to move quickly.

Most fish, to move through the water, perform undulating movements. These movements transmit waves from the head to the tail (caudal fin), driving the fish forward, overcoming the resistance of the water. Fish like manta rays use their broad pectoral fins as if they were wings, giving the impression of flying underwater. The eel, snakes (movement similar to snakes) to move. Many species of flatfish generate an interesting propulsive force. To achieve this propulsion, the fish swallows water through the mouth and expels it with force by the gills.

Swimming muscles

Fish that swim their whole life incessantly, must do so to keep their gills running. If they stopped swimming, they could die. To achieve this feat, these fish have two types of muscles: one for slow movements and another for movements that require speed.

These muscles are located on the sides of the fish, under the skin. One of them is the red muscle, rich in blood. This muscle allows the fish to swim at a constant and slow speed. This is so because this muscle supplies it with fuel in the form of oxygen and fat. While the fish is feeding and breathing, fuel will always be available.

The other muscle is white, with a smaller supply of blood and that provides the fish with glycogen (a carbohydrate). The white muscle is under the red muscle and its function is to provide the fish with an enormous amount of force when necessary, such as when it has to escape from danger or when it is going to catch prey. The limitation of the white muscle is that it can only be used in brief bursts. Glycogen, when decomposed, produces lactic acid, which if its use is prolonged, will cause fatigue in the fish system, which could leave it vulnerable.


I know this is a silly topic and not really relevant for most tropical fish keepers but I thought about it and wondered if they do indeed get tired of swimming. So, I thought it might be an interesting topic to research.

The long and short of it is fish do get tired and fish do sleep. They just don’t have eyelids like humans that they can close and fall asleep. They often create a home or nest/bed and rest to recover from a long day of swimming and feeding and chasing a mate.

Other questions always cross my mind like, Do fish Smell? and Can fish Hear? I know what you’re thinking, I have too much time on my hands or an interesting thought process. But other people have thought or asked the same questions as me because search results on Google prove it.  My Favorite is Do fish have good eyesight?  

I hope you enjoyed this fun topic and can now answer the question if it’s ever asked or pops up in your local pub quiz. Do fish ever get tired of swimming.

Happy Fishkeeping Forever!