Courses for Kids
Free study material
Offline Centres
Store Icon

How Do Fish Sleep with Their Eyes Open?

By Satabdi MazumdarSeptember 12, 2022
share icon

Fish Take Rest and Sleep: How Do They Do That?


Every organism needs to rest at some point. After a long day of survival, an organism will surely need rest to rejuvenate from the hard work and prepare for the next wave of survival. Fishes are also no different. They are primitive vertebrates that need sleep or rest to revive. A prolonged debated topic of ‘do fishes sleep’ has always fascinated us. Yes, they do but in a way different than we do.

Almost all animals sleep and fishes are no exception. Some close their eyes to sleep whereas most fishes do not have eyelids. Sleeping means closing your eyes and resting. Fishes, on the other hand, tend to sleep without closing their eyes as most species don't have eyelids.

What is Sleeping?

To understand the topic, let us first find out what sleeping is. It is a state where we suspend most of our physiological functions to rest and give all our active organs a time to revive. Sleeping is a unique state of body and mind where we enter our subconsciousness and enter a world of minimum activity and rest.

During this phase, we relax our muscles naturally. In fact, we release hormones that make our muscles inactive for 8 hours on average. We also inhibit our sensory activities and tend to cut ourselves off from the surrounding environment.

The same principle is applied to all animals that sleep or take rest. Fish do the same thing when they are sleeping. It is just that our sleeping patterns and practices are different. The environment we live in is entirely different from that of fish. So, the physical and sleeping activities will also be different too.

How Do Fishes Sleep?

The first question that arises in our minds is how do fish sleep? Most of the species do not have eyelids. The convention and definition of sleep suggest cutting off the senses from the external environment. How does a fish close its eyes to rest or sleep? Do they swim all the time or settle down on the earth-bed underneath the water?

If you look closely at the fishes in an aquarium, you will find they take breaks at regular intervals depending on their physical activities. It is during these intervals they lower down their physical activities and simply float to take a rest. Scientific logic also suggests that fish do not have eyelids and that means they can sleep anytime during the day or night. Only a few species of sharks have eyelids.

If you look at a sleeping fish, you will find out that they hover at a decided height at regular intervals but not with pace. It seems they are in a trance state. Most fishes do not have a proper place to rest. They might hide somewhere to avoid predation when they rest. Unlike other animals, most fishes do not set a place to rest.

An exception of Pacific Sand Lance, a unique fish will make you understand. This fish has developed a unique capability where it covers itself with sand to sleep underneath the marine waterbed and take a rest. This is how we can explain a specific fish sleeping time every species executes.

How Many Hours Do Fish Sleep?

You will be surprised to know that every fish species will maintain a proper sleeping pattern to rest. It can be during the day or at night. The sleeping schedule can be distributed across day and night in shorter segments. It all depends on their adaptations and survival.

Diurnal fish sleep at night whereas the nocturnal ones sleep during the daytime. These two types of fish have distinct physiological differences. These differences or physical characteristics can answer when fish sleep.

For instance, nocturnal fishes have bigger eyes as they need to see at night when the light is very dim. Vice versa, a diurnal fish will have smaller eyes compared to the nocturnal ones as they sleep at night and come out during the day.

The sleeping time and hours depend on the type of fish species. It depends on the time they need to rejuvenate their activeness by giving their organs proper rest.

Fishes Sleep

Fishes Sleep When They Need to Rest

How Does Fish Sleep?

Fishes do not sleep as we do. As mentioned earlier, they generally lower their physical activities and senses to a considerable extent. They then reduce the consumption of energy after finding a hideout or a safer place to avoid predation.

When they find a place safer to rest, they trim down their energy usage to a minimum and relax. They either rest on a surface or seem to hover at a particular level. Marine species with bigger brains even dream as we do. It is not proven that fishes dream but octopuses were witnessed changing colours when they dream.

Just watch any show fish in an aquarium and try to read its activity. You will find that when it is at its lowest energy level, it chooses a water level and hovers or rests closer to the surface of any object. This is how a fish generally sleeps.

There is a misconception that a shark has to swim during its sleep or it will drown. Sharks tend to rest on the seabed or in caves, cracks, crevices or together.

Fishes Do Need Sleep

If you watch an animated fish in any cartoon, you will find them sleeping. It is actually true. Different fish species have different ways of resting considering their breathing patterns and survival adaptation.

It is common that like all other animals fishes also reduce their biological activities to a considerable extent and become slow or stable when they sleep. Their response time also increases considerably. Hence, we have learned that fish sleep but in different ways than the other animals do.