September 9

Do Fish Sleep With Their Eyes Open? – All the Information You Need


It can be challenging to understand a fish’s attitude or behavior, particularly for those who are new to keeping fish as pets. Fish species also sleep very differently when compared to animals or humans.

So, let us discuss all fish sleeping. Along with discussing these common questions: Do Fish Sleep With Their Eyes Open? Or Do Fish Keep Their Eyes Open When They Sleep?

Let’s begin…

Do Fish Sleep With Their Eyes Open? 

To put it briefly, yeah! They can sleep. Their sleeping method is a bit different from ours (humans), but they are able to sleep at any time, whenever they want. However, their sleeping patterns vary from one specie to another.

How Do Fish Sleep?


Most species of fish can sleep by making themselves inactive or dormant. Their overall activity becomes less, and they try to find some hiding spot or the corner of the aquarium that they find comfy (in the case of pet fish). 

Pet fish can also slow down their metabolism in their sleep. However, Wild fish, on the other hand, are able to maintain awareness of their environment and remain vigilant for potential dangers even while they are sleeping.

Some fish may take refuge in nests, while others will slither into cracks in the rocks, and yet others will burrow into the substrate in order to get some sleep without fear of being disturbed.

But remember, some fish, including some sharks, constantly swim while “sleeping.” This is very necessary in order to maintain a constant flow of water over the shark’s gills. If the shark were to cease swimming, it would, in all likelihood, eventually die. 

Do Fish Sleep? Or Do Fishes Sleep?

Yes, they do sleep. In scientific language, these phases of “sleep” in fish are more accurately referred to as “suspended animation.” It is because suspended animation is referred to as periods of rest in which the fish recover from the previous day’s efforts and recharge their brain and body in a manner that is analogous to what other creatures do while they are sleeping.

sleeping fish

Do Fish Have Eyelids?

The large majority of fish don’t have eyelids since they do not have a physiological need for them. But certain species like sharks and milkfish do have eyelids.

Our eyelids’ most important job is to prevent the eyes from drying out by trapping moisture in the eye sockets. In addition, our eyelids shield our eyes from potentially harmful materials present in the air, such as pollen, dirt, and other minute particles. This helps to keep our eyes healthy.

On the other hand, fish are not subject to any of the aforementioned challenges. Because they are continuously present in the water, there is little risk that dust or sand particles from the air will get into their eyes, nor do their eyes ever become dry because of the constant exposure to water.

Tip: Some fish have a simple membrane that covers their eyes for protection, whereas others have a layer of fatty tissue that acts as an eyelid.

How Do You Know If A Fish Is Sleeping?

When fish are sleeping, it’s relatively easy to tell since they lie motionless, most of the time around the surface of the water or at the bottom of the tank. They take a long time to react to things that are happening around them, or they don’t react at all sometimes (as discussed above).

    If you observe the movement of their gills, you will discover that they have an extremely sluggish breathing rate.

    Those who keep fish in tanks at home are likely aware that the aquarium fish become much less active when the lights are turned out at night. You’ll be able to observe how motionless they are in the midst of the night if you switch a light on for yourself.

    Fish, much like people, are equipped with an internal clock that informs them when it is time to perform activities like feed and sleeping. Therefore, even if you make the mistake of leaving the lights on during the night, the fish may still calm down and fall asleep anyway.

    Do Fish Sleep On One Side?

    Sleeping fish don’t roll over or turn sideways; they stay in the same position during their sleep. A fish that appears to be on one side, is upside down, or is laying on the bottom is not resting; rather, it is most likely ill.

    So, the one-word answer to the question, do fish sleep on their side, is “No”. But remember, never confuse it with the side of the aquarium/tank. Fish may sleep on one side of the tank where the place is more comfortable, especially the area with dim light and hiding spots.

    How Long Do Fish Sleep? 

    do fish sleep

    Well, that is dependent on the type of fish. In order to maintain their normal functioning, the majority of fish species require between eight and twelve hours of sleep on a daily basis, on average. Some species are known as nocturnal fish, meaning they are most active at night.

    These fish, which are active at night, spend the daytime hours sleeping in cracks, holes, or other places where they may conceal themselves from predators.

    It is advised that the lights in your aquarium be turned off overnight, as most fish that are diurnal will not be able to tell the difference between day and night (if the light is on).

    Turning off the lights in the aquarium will signal them that it is nightfall and that they must go to sleep immediately. It will make it easier for them to maintain a consistent sleeping schedule.


    Do Fish Close Their Eyes?

    No, most fish, especially those with eyelids, cannot close their eyes. 

    Which Fish Sleep With One Eye Open?

    Dolphins can sleep with one eye open. They do this to keep them aware of the predators/threats while sleeping. Experts believe this phenomenon is particular to dolphins, sometimes called unique sleep habits.

    How Are The Eyes Of Fish Different From The Eyes Of Humans?

    The main eye anatomy of fish is very similar to those of humans. The lens of each eye is the only different thing. However, there are two main differences:

    1. To better see in bright conditions, humans have flat and relatively thin lenses. Fish, on the other hand, inhabit environments with less available light. Their vision is enhanced by the thick, spherical lenses found in their eyes.
    2. Another distinction lies in the fact that human eyes are protected by eyelids that may blink. It may not appear significant, but the eyelids actually perform a crucial job! They maintain the moisture in our eyes and also assist protect them from the dust that is in the air. On the other hand, fish do not have eyelids since their environment is highly humid and does not contain dust.


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