Home » Fishkeeping FQA » How Do Fish Sleep? With Open Eyes | Mystery Solved

How Do Fish Sleep? With Open Eyes | Mystery Solved

Do fish sleep? This may be one of your most curious questions from childhood. The answer is simply Yes! Fish do sleep. Like any other animal kingdom member, fish verities also need proper sleep to maintain their health. Sleeping and resting is an essential needs for any animal. It helps the body recover from any damage and especially to repair and maintain the brain’s right working conditions. But, the question is, how actually do the fish sleep?

What is sleep?

First, we need to get an idea of what sleep is. If you ask the same question from anyone in the world about what sleep is, they may just answer the eye closure with a low level of activity is sleep. Indeed the land animals like you and me; close our eyes to get asleep.

All other amphibians and land animals and birds all close their eyes to get sleep. But, fish does not sleep that way. As they don’t have eyelids, how could they close their eyes? So, how do they sleep? Let’s find out the best answer to this “most wanted Question.”

How do fish sleep?

Science has given a definition of sleep. If any activity is sleep, it has to fulfill two requirements.

  • The first thing is eye closure.
  • The second requirement is the presence of electrical brain activity patterns in the brain.

In mammals, these activities take place in a tissue called the neocortex. Applying this definition to fish is that the fish does not have either the neocortex or eyelids.

A scientific explanation

Still, the science does not have a fully explainable complete answer to the sleeping behaviors of fish. Some scientists demand, that some fish verities never sleep (even if they sleep, there will be no sign of sleep).

For example, if we consider fish verities living in the fast-moving waters, they always have to swim against the water current to maintain their body balance.

Aquarium tanks at night

But, if you look at your home pet fish tank at night with low or no light, you may notice that some of your fish are unresponsive and motionless.

But, you will not see any fish lying on the bottom with their eyes closed. You already know why. That is because they don’t have eyelids. (Only some cartilaginous fish verities such as sharks have eyelids. But, the purposes of those eyelids are to protect their eyes during encounters.)

How do you know if the fish is sleeping?

Briefly and commonly, when fish sleep,

  • Their metabolism rate will come down.
  •  They will show less or no motion.
  • Some verities will hide in dark caves for sleeping.
  • Some verities become motionless in the dark (Zebra verities, Tilapia verities, Doctor Fish, etc.)
  • There are some verities like Tuna, and some sharks who do not show any sign of sleep at all.

Sleeping fish is difficult to arise.

Their level of attention to the environment is very low. You can especially notice this in your home fish tank. If you tap on the glass, the sleeping fish will take some time to notice the noise.

But, in the wild, the situation is a little bit different. Since the wild waters are really dangerous, most fish verities in the wild stay alert for any possible danger while sleeping (sleep deprivation), so a little motivation will remove them from the resting location immediately.

How many hours do fish sleep?

How many hours do we sleep? Nature is always perfect. It taught all the organisms to work during the day, fulfill hunger during the day, and rest at night. Even the plants rest during the night. When the daylight is not present, the brain automatically signals Hey fellow; now, it’s time to sleep.

Diurnal fish

Likewise, like any other animal, most fish verities have the same sleeping behavior. Most fish verities cannot see at night. So, the only option they have is to stay still and have some rest.

We call this behavior diurnal (Work during the day and sleep at night). This behavior is very significant in aquarium fish verities. Since they do not have natural predators in the same ecosystem, their peaceful sleeping behavior can be easily observed during the night.

Nocturnal fish

However, some fish verities do the opposite. Think about a thief. What is the best time for him to go and break some home? Well, you know the answer. The thieves know that during midnight all other people fall into a deep sleep.

No one feels that a thief is collecting their valuable things just a few inches away from them in this state. Some fish verities have the same behavior as these thieves. They are called nocturnal fish verities.

Diurnal fish rest during the daytime. When the darkness arrives, they start their work not to steal gold or money. But to steal the most valuable things that other fish have in their lives.

When do fish sleep?

The answer is not obvious, and it depends on the environmental conditions, fish variety, and many more things.

  • Night – Most fish verities, especially aquarium fish, sleep at night. Since they do not have a good ability to see in the dark, they are made by nature to sleep at night. Some fish verities find a hiding spot for optimized darkness for sleeping.

E.g.:  Betta fish – According to research, betta fish need 12-16 hours of a dark period to keep good health. If you place a bright light with no dark place to hide in the tank, your betta may get stressed. Most common aquarium fish verities sleep in the dark. So, they need a proper dark period.

  • Day – While the others are sleeping, some predators with high sensing abilities hunt down the sleeping fish at night. The most common and powerful predators, such as shark verities, show this behavior.

E.g.:- Most invertebrates kept in aquariums sleep during the daytime and get activated at the night. Also, fish verities like eels and other aquarium animal verities like octopus have the same behavior. So, do some research before choosing a fish verity for your aquarium tank and plan the lighting very well.

  • Not sleep at all – This is not actually a fully proven fact. But, scientists have reported that some fish verities do not show any sign of sleep at all.

During migration and mating periods, most fish verities do not sleep at all. Some fish verities may sleep while swimming and do not show any sign of sleep at all.

So, if you are an aquarium keeper, give your fish some dark time to complete their sleep. Otherwise, they may get stressed easily. So, always keep in mind that you and I, even the fish, need a dark period every day to rest. Think twice before installing high bright lights that run 24/7 for your fish tank.

Types of fish sleep

Not all fish verities sleep the same way. Some may find a hiding spot to sleep and to protect from predators while sleeping. Let’s go through some interesting facts about fish sleep.

Fish sleep while floating

Betta Fish – Bettas sleep while floating. You may notice that your betta is dead and floating near the surface. But, he is totally fine and just sleeping.

Fish sleep upside down

No aquarium fish sleep upside down. If your fish is upside down, that may sign a disease such as a swim bladder disorder.

Do Body color decrease while sleeping

Guppys‘ body color decreases while sleeping. Bright body colors may attract predators. So, in most bright fish verities, the body color decrease while sleeping.

Do Fish sleep in cocoons

Parrotfish create a cocoon for sleeping. It acts as a shield. Any predator has to first break the shield to reach the parrotfish. So, he has enough time to go away.

Do Sleep while swimming

Some sharks sleep while swimming. Sharks need a continuous supply of water through their gills for breathing. So, most shark verities must swim to breathe. They will drown if they stop swimming because they rely on ram ventilation.

do fish sleep while swimming

The sleeping stages of an aquarium fish

Still, science does not have a full explanation for this question. However, some researchers explain the sleeping stages of aquarium fish. According to the USA’s national ocean service, fish sleep is something like reduced activity and metabolism while remaining alert to danger.

Research by the front neural circuit revealed that the zebrafish (Danio rerio) sleep cycle has a huge similarity to the human sleep cycle. According to this research following points have shown significant equality in human and zebrafish sleep cycles.

  • Human and Zebrafish sleep ratio decreases with age.
  • The human and zebrafish wake ratio increases and awakenings at night lengthen with age.

Another research on the same species showed that zebrafish larvae also have a rhythmic sleep-wake cycle as humans have. So, even with the limited research materials we have, we can imagine that most fish have the same sleep-wake cycle as humans do.

What do fish do when sleeping?

It depends on the fish variety. Some fish verities like goldfish do not enter a fully relaxed state like we, the humans, do. When we sleep, our body stops almost all movements, and during the deep sleep stage, the body enters a very deep state where the attention to the environment becomes very low.

But, when sleeping, the goldfish will still show some movements, and they will stay alert to the signals coming from the outer environment.

Can fish have dreams?

Some researchers have proven that fish can dream like you and me. A video released on youtube shows an octopus changing color as he dreams.  Octopus can change their body color to hide from predators. But why would an octopus want to change body color while sleeping? Scientists demand that the octopus dream about a predator, and his body thinks it is a real incident. So, body-color change accordingly. 

Do fish have REM sleep?

No, fish does not underage rem sleep. Also in fish, there was no report of rapid eye movement or muscular twitching that are characteristic of REM sleep in mammals(eye movement).  They definitely sleep, some probably sleep one hemisphere of the brain at a time. a good example is a tuna.

Does it help to repair damaged body parts?

When sleeping, the rate of metabolism will decrease gradually. So, the body can repair and regenerate any damaged parts. When you get sick, what is the best thing you want to do? Or what does your brain ask you to do? Sleep right?

The same happens with the fish verities. When fish get ill, they will try to sleep a lot, and their activities and movements will be limited. It happens because only sleep can help a body cure itself. This nature’s law is common to you, me, and also to your fish. Before sleeping, they may find a safe place to hide from predators. So, in the wild, you may not see any sleeping fish because they are already hiding and adapting to the environment to avoid predators.

Where do fish sleep?

It also depends on the fish variety and environmental conditions. Especially if the oxygen levels are low, almost all the fish will come to the surface of the water to get enough oxygen. But, for each and every species, the sleeping place and sleeping habits vary. 

Fish sleeping at the bottom of a tank

Each and every fish prefer a unique environment to live in. Some fish like guppies, Swordfishes prefer to live on a tank’s surface while fish verities like betta prefer the medium tank layer. Catfish verities are bottom feeders. So, they prefer to live at the bottom of the tank.   So, if your fish is lying on the bottom of the tank, remember he or she is not sleeping all the time. It may be their natural, normal behavior.

Fish sleeping habits matter

But, your betta or guppy fish may go to the bottom of the tank for sleeping. Some fish verities choose a place in between gravels to sleep. Also, some may need a cave-like dark place to sleep.  So, no matter if they are bottom feeders or surface fish, they may go to the bottom of the tank for sleeping. You may be a doctor, a driver, or an IT professional, but you don’t sleep on your office desk or car, right?

Like we have beds, some fish may also love to make their own bed. This bed not only makes them comfortable but also will save them from predator attacks. 

Do fish sleep at the top of the tank?

If your fish is trying to sleep at the top of the tank that may be a sign of a lack of oxygen in the tank water. Fish cannot intake oxygen from the atmosphere as we do. They can only extract the oxygen amount, which is dissolved in the tank water. So better check your water condition.

Water quality may affect fish sleep.

If your tank is dirty, the fish may lack oxygen. Your fish will come to the surface of the water to sleep. So, it can take the required oxygen amount without threatening its life.  If the tank oxygen levels are low, some fish verities like guppy will try to jump out from the tank. So, make sure you maintain a good livable condition in the tank. 

When the nitrogen waste levels in the tank are high, the fish will easily get stressed from it. So, they won’t be able to sleep. What happens if you miss a few days of sleep? The same will happen to your fish. So, always be careful. 

Fish sleep upside down?

Nope. They will never sleep upside down unless they are dead or suffering from a disease like a swim bladder disorder.  Swim bladder disorder is very common among Carp, goldfish-like fish communities. The swim bladder is an organ filled with air, which helps the fish to maintain an upright position. But infections may overload gas into the swim bladder. So, the fish will get upside down. Unfortunately, there is no perfect cure for this disease. Only the immunity system of the fish can beat this disease. 

There is a clear connection between water quality and swim bladder disorder. So, always keep your tanks clean and clear. 

Do fish suffer from sleep disorders?

Yes, sleeping sickness or sleep disorder is a response to extreme stress levels caused by a viral infection. This sickness is common in koi, carp, and goldfish populations.  The fish will suffer without sleep. So, stress levels will increase rapidly when sleep patterns change.

They will avoid eating and will try to avoid any movements in the tank or pond. Unfortunately, this is an incurable disease. Only the immunity system can help the fish to recover. But, only very small amounts of fish are lucky to recover from this disease. If you see any infected fish remove it from the tank to avoid the disease spreading to others.

Do fish tank mates affect fish sleep?

Indeed yes. The presence of aggressive fish types in the same tank will affect the sleep of other fish. For example, betta male fish sometimes get aggressive. When they are aggressive, they chase other fish and try to attack them. Betta male marks their territories in the tank. They hate to see any movements in his area. 

So, other fish may get beaten by the betta. So, you have to consider the aggression level of each fish before choosing tank mates. Catfish verities, some gouramies, Barb verities, Knife fish, shark fish, and cichlid fish are considered aggressive. (There are more. So, do some DIY research before purchasing them)

Bullying affects sleeping in fish.

If we consider a guppy population, the males always try to chase female fish. So, if there are more males than females in a tank, the females will easily get stressed and won’t be able to sleep at all. So, consider introducing fish with a proper ratio and preparing some hiding spots in the tank. 

Bullying affects sleeping in fish

How Do I Make Sure My Fish Get Enough Sleep?

Maintain very good water quality in your tank

Do regular water changes and track chemical levels of the water. Do not do full water changes frequently. That can also stress your fish. Try doing a half or quarter water change once a week. You can buy water quality indicators for a very cheap price online. Keep an eye on the nitrate levels in the water. 

Turn off the lights at night
Give the fish some dark time to sleep. You can set up a timer to turn off the switch of your tank lighting at night. Keeping the lights on without a break can easily stress out the fish. When your fish get stressed and lose sleep, they are more vulnerable to diseases. 

Introduce hiding spots

Some fish verities love to sleep in a hiding spot. Driftwood is probably the best hiding spot you can provide your fish. Natural driftwood is a great add-on to your tank. It will provide beauty, and also some wood verities like Indian almond is capable of balancing the pH in the water. 

Avoid stress 

Having a big tank with some personal space and good food will be important. To avoid stress in fish, providing a good meal and perfect environmental conditions are very important to upkeep good mental health in your fish. If your fish get stressed, then they will lose sleep. When they do not get enough sleep, they will probably get ill very quickly.

Good food

Who can sleep in hunger? Provide a good healthy meal every day. Consider providing a live meal when possible. Like humans, fish also need proper nutrients. This includes vitamins, proteins, and fibers. Usual cheap fish food will not fulfill all these requirements. So, do some research on your fish nutrition requirement. 

Choose tank mates wisely

Aggressive tankmates will not give your fish a chance to live peacefully. So, when choosing tank mates, make sure not to choose aggressive ones. Also, keep the balance in the tank. Overcrowding will make your fish very aggressive. So, choose a tank with plenty of space. So, your fish will have a good personal area for resting. 

Avoid installing high bright lights.

Keeping your tank in complete darkness is also not good for your fish. Also, if your tanks have plants, for sure, they need light for photosynthesis. too bright light can cause sleep deprivation.

How Do I Make Sure My Fish Get Enough Sleep?

Good temperature

If your tank is too cold, your fish may not get proper sleep. Also, they will get ill within a short time. So, introducing a proper heating mechanism to the tank will be helpful for your wish to rest well. 

Can fish sleep with the light on?

No, they can not. Fish need darkness to properly complete the sleeping cycle.  Your tank lights or other lights pointed at your fish tank should only be on for 10-12 hours a day even under the moonlight settings. it can be harmful to fish in long term. due to lack of sleep, your daytime fish will be stressed, and your night fish will also be stressed due to light. This will keep your fish happy and safe.

Also, the duration of sleep depends on the fish species and other environmental factors. light plays the main role. In aquariums, the sleep cycles of fish are controlled by interior lighting. When the lights are switched off the fish will sleep. Many fish are active in the daytime and sleep at night, such as minnows and coral reef fish, while others do the reverse and are active at night instead. So bellow table will help to find

How Much Light Does Your Aquarium Need?

Stocking LevelHours Of Light
Tropical Fish8-10 Hours Daily
Tropical Fish & Plants both10 Hours Daily
Heavily Planted Tank12 Hours Daily
Cold water Fish & Plants8 Hours Daily
Cold water Fish Only8 Hours Or Less Daily

Can fish sleep with a blue light on

Yes they can, The blue light helps fish to identify day and night time because it imitates moonlight in an aquarium. So blue light does not impact the sleep cycle of fish or plants and does not contribute to algae development. It can be a great addition to any tank. Fish are free to swim around or ‘sleep’ at night which is more like a time of rest than a true slumber.

But you need to keep in mind, that even blue light calms your fish and transitions them by mimicking moonlight keeping them on for the whole night is not good for your fish. because most of the fish require a few hours of darkness every day. it is better if you can use an aquarium light timer. This way, you can ensure that your aquarium fish get their required dark hours. under normal conditions, 2 hours of blue light is recommended for fish.

How to choose a bulb or light for your aquarium

But choosing a bulb with high brightness will probably make your fish blind. So, do not go for a high bright one just because they look so cool. Fluorescent bulbs are decent. They produce attractive colors. But, they may generate excessive amounts of heat, which may increase the tank water temperature to bad levels.

Too much lighting can also increase algae growth. Like plants, algae also photosynthesis to produce food. So, your tank may get green color too fast if you choose a high bright light. 

Types of Aquarium bulbs

Many types of aquarium bulbs are available in the market. But, not all of them are good for your fish tank. Normal fluorescent bulbs, High output bulbs, Very high output bulbs, Power compact bulbs, and metal halide bulbs are the most common fish tank lighting systems available in the market.  

Normal fluorescent bulbs

They produce enough light for fish-only setups. The brightness is well enough for fish. It does not harm your fish. Also, they do not produce excessive heat. So, these bulbs are highly efficient for fish tank lighting.

High output bulbs

The efficiency is somehow low. So, they produce some heat compared to the fluorescent bulb. But, these bulbs are good enough for small tanks with plants. 

Very high output bulbs

The light intensity produced by these bulbs is really high. It is very good for a planted tank. But, the fish may find it somehow uncomfortable. 

LED bulbs

Most aquarium keepers prefer LED bulbs for their fish tanks. LED bulbs are efficient and durable, and the light is not as harmful as other bulbs. Also, they do not produce excessive heat amount. Choosing the correct light intensity and wattage is important when choosing a LED bulb. 

Learn more about aquarium lights


Like humans need good sleep to maintain a healthy lifestyle, fish also need proper sleep. Some research demands that the fish also have the same sleeping cycle and a habit as humans do.  Fish also can dream. They can even move while sleeping. For some fish verities, zero or no movement is not present while sleeping. Likewise, humans need good sleep to upkeep healthy fish do need a good night’s sleep to maintain their body Light, nutrients, and many things that affect their sleep. 


Source research paper 1

Source research paper 2


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About Dr.Chamika

Hello, I'm Dr. Chamika. I am a Researcher in Water quality, Aquatic organisms, and Environmental chemistry. I am a passionate fish keeper, with10 years of experience. My mission is to help other aquarists experience the joy of fish keeping.