It can be hard to read a fish’s mood or behavior, especially for a first-time fish keeper. They don’t make very many facial expressions like humans do, nor do they have the expressive vocalizations of dogs and cats. They have their own methods of communicating with one another, but these don’t translate well with people. The end result is that it can be hard for a beginner aquarist to tell what their fish is thinking or doing, or even if it’s sleeping, sick, or dead. Once you’ve gained a bit of experience, though, the distinctions between all of these behaviors become easier to tease out. In this article, you’ll learn more about how and when fish sleep, and of course, we’ll also answer the all-important question: Do fish sleep with their eyes open?


Do Fish Sleep with Their Eyes Open?

            The short answer here is yes, and the short explanation for why is that fish don’t have eyelids. They physically cannot close their eyes, whether they’re sleeping or not. In the context of their environment, this makes sense. Human eyelids help keep the eyes moist and protect them from dust and other debris that may enter the eye. Fish obviously don’t have to worry about keeping their eyes moist or protecting them from dust. There is also significantly less light in most bodies of water, particularly the ocean, than on dry land, so fish are less likely to need to close or cover their eyes to protect them from the sun.

            Additionally, because light and visibility are so limited underwater, fish do not use vision as their main method of understanding or navigating the world. Underwater, when a meal or a threat can come from any direction, it would be impractical to rely on a necessarily limited field of vision to find food or dodge predators. Most fish instead rely on smell, sound, and lateral lines, which sense movement in the water, to understand the world. Shutting their eyes, then, even if they could do it, would not have the same effect as it does for humans, of shutting out sensory information and allowing the brain to rest.


Do Fish Sleep?

            After watching your fish for a while, you may begin to wonder if fish sleep at all. Some species are known to rest on the bottom of the tank or in caves or corals, but most species never seem to take a physical rest, always floating or even swimming in the tank. This isn’t because fish don’t ever sleep, but they don’t really sleep in the way most humans and mammals do. Many ichthyologists prefer to refer to fish resting, rather than sleeping, because the mechanisms of the behavior, as much as we know what they are, are so different from what we understand as sleep as to not qualify as the same process.

            Of course, there are thousands of different species of fish, and how each one rests is different. Sharks, famously, never stop swimming, even when resting, because their gills require water constantly flowing over them to keep the shark oxygenated. Many species adopt this method of resting their brains while their bodies keep moving, albeit slowly, while others shut down more and simply float in the water. Others nestle themselves in a safe place before resting, potentially hiding in a cave, plant, or on or in the substrate.


How to Tell If Your Fish Is Sleeping

            How a fish rests will depend on its species and environment, so make sure to research a fish’s general behavior and resting patterns before adding it to your aquarium. All fish keepers should watch their fish closely and develop a sense of their patterns of behavior because the best way to tell if a fish is ill or simply resting is to know if this is a common behavior for them. In general, though, a resting fish will continue to have regular gill movements, even if they’re slower than normal, and will remain alert enough to their environment to move if something gets too close or disturbs them. A fish that is bumping into things, not moving at all, or seems to be resting a lot more than normal is likely to be ill and should be isolated, more thoroughly examined, and potentially seen by a vet.


Unlike dogs and cats, fish are so biologically and evolutionarily different from humans that it can be hard to tell what’s normal behavior for them and what is a cause for concern. Don’t worry if you never see your fish sleeping; they’re likely resting out of sight, or while continuing to swim. Once you get to know your fish well, you’ll be able to easily tell when they’re ill, active, or resting, and can spend more of your time sitting back and enjoying the show.

Are there animals that do not sleep?

No. All animals have something like sleep, even insects. The lower animals with little or no brains sleep differently from humans, but they exhibit periods of inactivity when they are less responsive to external stimuli.

In fact, research with fruit flies has shown some of the same biochemical action in them as happens in human brains during sleep. The commonality points to how ancient sleep is in evolutionary terms. Sleep is universal and needed by ALL LIVING CREATURES…including by YOU! If you are having sleep issues, Alaska Sleep Clinic is here to get you back on track.

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