Do Fish Sleep With Their Eyes Open?

Fish sleep with their eyes open because they lack eyelids. They also keep moving while sleeping, which sounds tiring. Let’s learn more about fish sleep.

Apr 24, 2024By Monika Dimitrovska
do fish sleep with their eyes open

If you’re a first-time fish owner, you probably have a lot of fish-related questions.

One of them is probably: do fish sleep with their eyes open? The short answer is yes, fish sleep with their eyes wide open, but they don’t have much choice because they lack eyelids.

In this article, we’ll explain how exactly fish sleep and share care tips for first-time fish keepers, so let’s dive right in!

Do Fish Sleep at All?

goldfish fish bowl lights
Image credit: Ahmed Hasan from Unsplash

Most fish sleep, but some, like blind cave-dwellers and deep-water varieties, don’t because they don’t have much to process, unlike other fish. Instead of sleeping, they keep swimming.

However, as I already said, most fish sleep, but they don’t do it to recharge like humans. Fish actually have periods of lowered activity. Additionally, some fish rest at the bottom of the tank (that’s how fish owners can tell their fish is resting) or in caves. Others keep moving slowly or find safe spots to rest.

NOTE: Fish dwelling at the bottom of the tank may indicate resting behavior, but it could also be a sign of common fish diseases, such as parasites, GI problems, stress, etc.

Fish dwelling may also signify improper water conditions (very low water temperature), so make sure you maintain a well-balanced aquarium for your fish.

How Do Fish Sleep?

yellow pet fish hiding
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Fish make fantastic pets and have unique sleep habits, but they don’t sleep like other animals or us. In fact, sleep in fish depends on various factors, including light exposure, physical traits, social hierarchy, and environment.

Some (rainbow wrasse and zebrafish) sleep at night like humans, while others (sharks) rest throughout the day because they’re nocturnal. Additionally, cave-dwelling fish don’t need as much sleep as surface-dwelling ones, like Mexican cavefish.

Fish even breathe differently during sleep, depending on the species.

Some bony fish, rays, and sharks can remain still while breathing through their gills by opening and closing their mouths. Others must keep moving to maintain water flow over their gills. So, they either swim slowly or sleep with part of their brain active.

Do Fish Sleep With Their Eyes Open or Closed?

pet fish hiding
Image credit: James Lee from Pixabay

Fish sleep with their eyes wide open because they have no other choice. Most fish don’t have eyelids, so they can’t close their eyes when sleeping. They also keep moving even while sleeping, which sounds exhausting.

However, we must understand that fish don’t experience sleeping as we do. They gently float and occasionally adjust their fins for balance and oxygen while resting.

While some fish sleep at night like us, most prefer to sleep during the day or find safe hiding spots. And despite being asleep, they stay somewhat alert to escape predators if needed. As you can see, fish have adjustable sleeping patterns.

Their resting schedule depends on water temperature, food availability, predators, and even parenting, just like new human parents adapt to caring for their babies.

How Do You Know if a Fish is Sleeping?

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If you’re a first-time fish owner, you won’t be able to tell when your fish sleeps because even when they’re resting, they’re still on the move (they must keep moving to breathe underwater). They also have their eyes open at all times, which makes it even harder to recognize if they’re sleeping.

However, as time goes by, you will learn their habits and behaviors. Most fish, including fish breeds for first-time owners, exhibit the following signs when sleeping:

  • Staying still for a long time
  • Resting in a particular spot, like a hiding place
  • Floating gently to breathe through the gills
  • Not reacting much to outside disturbance

Hiding is often regarded as one of the strange behaviors in fish. However, given that both big and small fish have many predators, it’s understandable why they would hide when resting.

Can Fish See When They’re Asleep?

clownfish aquarium
Image credit: Silvo Bilinski from Pixabay

Fish don’t experience REM (rapid eye movement) sleep but have periods of rest where their activity levels drop. During these periods, they usually reduce their movements and find a quiet spot in their environment.

Moreover, some fish species have a nictitating membrane or a transparent eyelid-like structure that can cover their eyes. This membrane can provide some protection to their eyes and may reduce sensory input during periods of rest.

Therefore, even though fish don’t “see” in the same way as us when resting, they might still have some level of visual perception. However, this depends on various factors like species and environmental conditions.

How Do Fish Sleep While Moving?

colorful fish swimming
Image credit: Anja from Pixabay

Fish, including some of the best freshwater tank cleaners, move less when they sleep but move nonetheless because they need oxygen from water flowing over their gills.

However, certain marine mammals and bird species use a special sleep method called unihemispheric sleep, where one half of their brain rests while the other stays awake. This way, they can swim slowly while still getting some rest.

That said, it’s safe to assume that if all fish had eyelids, one eye would close while the other stays open, similar to some birds and marine mammals.

Final Thoughts

yellow fish aquarium nighttime
Image credit: Krys Amon from Unsplash

In a nutshell, fish sleep with their eyes wide open because they don’t have eyelids to close. However, they’re built this way because they must stay somewhat aware of their surroundings during sleep.

Understanding the basic fish anatomy and their resting habits can help fish enthusiasts, especially first-time fish keepers, take better care of their aquatic companions.

In other words, by observing your fish closely and learning their behavior, you can easily recognize signs of illness and ensure their well-being. And with this knowledge, you can live in harmony with your fish for much longer.

Monika Dimitrovska
By Monika Dimitrovska

Monika is a pet enthusiast and seasoned copywriter with a tech degree. She loves writing, but her heart belongs to her two mixed dogs, Buba and Bono, a mother-son duo. Bono’s siblings found loving homes, sparking Monika’s advocacy for neutering and deepening her curiosity about animal care.

But Monika’s pet family doesn’t end there. She also has two cockatiels and two rescue cats, proving her home is a haven for creatures big and small.