Do Fish Sleep With Their Eyes Open or Closed?

Do Fish Sleep With Their Eyes Open or Closed?

That is a good question because when you look into your aquarium, the fish seem to always be swimming around. They never seem to stop. Do they swim and sleep at the same time? Or do they even sleep? The answer is yes, they do sleep but it is with their eyes open. That is why you never know if they are sleeping or not.

Do Fish Sleep?

Yes, fish need sleep just like humans do. If they cannot sleep, then they could suffer from health issues.

  • Why: A fish sleeps to recuperate and rest. If they have had an injury or an illness, they use sleep to help them recover. A diurnal fish has a sleeping pattern similar to a human. They sleep at night and work during the day. Fish do not dream while they sleep because they do not have the neocortex, which is the part of the brain that helps you dream.
  • How: A fish becomes inactive when it goes to sleep. Depending on their situation and the other species around them determines the extent of their inactivity. Some species will become completely inactive and will then float in the same place until it wakes up. Some will be mildly inactive and just circle the aquarium while sleeping. 
  • When: For some fish, they will go into hiding so they can rest. There are some nocturnal fish that will hide during the day and come out at sunset.
  • How Long: This depends on the species because some fish do not sleep at all. There are freshwater fishes that live in lakes and rivers that sleep for different time durations. As the seasons change so do their sleeping patterns. Some fish will sleep twice as much in the winter as it does during the summer.

How to Tell Your Fish is Sleeping

Because of their monotonous way of swimming it can be difficult to tell if they are asleep or not. Here are some tips to help you identify if your fish is sleeping.

  • Slowly Swimming: The speed at which it is swimming is slower
  • Insensitive to Stimuli: Your fish is not be as alert to its surrounding
  • Sleeping Time is Fixed: The fish will usually fall asleep at night.
  • Color is Faded: When the fish is asleep their color will subtly fade.

Sometimes they will have a special resting posture or they will sleep at the bottom of the aquarium. Some even cover themselves in mucus. Just look for a fish that is zoned out and floating; that is your fish sleeping. In the wild, some fish will find places to sleep like underneath ledges, buried in the sand, in crevices and holes, and amidst vegetarianism.

Why Do They Move When Sleeping?

As it has already been established, most fish will sleep differently. Some will hide while others will continue to move. When they move while they are sleeping this is to keep the water moving past the gills. This is done to help maintain their oxygen levels so they can keep their body stable. Some fish will flip on their side when they sleep because they do not have a pelvic fin. Other fish have gills called spiracles that let the fish stop swimming when they are resting.

The little fishes also need to sleep being slightly aware so it has to keep swimming. Being aware just a little can mean the difference between death and life. The shark is able to shut down parts of the brain so they can rest while swimming.

Do All Fish Sleep the Same Way?

All fish species of fish sleep differently.

  • Lie Down: A fish will swim to the bottom of the aquarium to rest. You might think it is dead because of the way it is laying but it is only playing dead.
  • Drift: If you notice your fish just cruising aimlessly through the waters it is most likely sleeping. The tail or fin might be moving but it is asleep.
  • Burrow: This is like we do when we use a blanket to cover up. The fish may sleep in the reefs or toy cave in their aquarium.
  • School of Fish: A school of fish is sort of like a pack and when some sleep the others will be alert for predators. They all take turns sleeping and being awake.
  • Shut off one Part of Their Brain: A dolphin will shut off one hemisphere of the brain. This will provide them with rest and let them go into a state of sleep. This is called hemispheric slow-wave sleep. The part of the brain that is awake will keep track of things around them and continuously swimming


  • If you have a fish that goes inside a reef or something else in the aquarium to sleep when it gets dark and then it is inactive until daylight, you should get a timer to regulate the fish tank light. This will help to make sure that your fish gets enough exposure to darkness and light.
  • Fish do not go into a deep sleep because if they did, they could become a meal for a predator.
  • No matter how they sleep, in the wild, a fish is always on the alert for predators. If it even senses one, they will instantly wake up and swim away.
  • A goldfish will float somewhere near the bottom with their head pointed down slightly, and their fins spread out and relaxed when they sleep. 
  • When a fish is awake during the day and sleeps at night, they are considered a diurnal fish.
  • When your fish is sleeping, do not startle them by trying to feed them, make any sudden movements around the aquarium, or switch on lights. It can cause undue stress on your fish from not getting the right amount of rest.
  • The reason that fish sleeps with their eyes open is that they do not have eyelids. Sharks are the only one that has eyelids.

Also, check out: Will the Tail or Fin of a Fish Grow Back?

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