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How do fish sleep: Something We Don’t Know!

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Hello Gnees Army,

Good Morning,

Do you like to sleep? Wait!..what? what are you saying??? Are you talking about sleeping? Ohh! I’m already a grandmaster in that role 😆 😆

Getting out of the joke, most of us love to sleep. Now imagine, when it’s close to your bedtime but you couldn’t lie down, close your eyes, and sleep on into your warm, comfy bed. How will you feel? Would you still want to spend your time sleeping? Would you even be able to sleep?

Hmm… Of course – No, unless you’re a SUPERHERO! Now, what about fishes in sleeping? If you watch them carefully, you’ll find that they’re constantly swimming around. It can seem like they never stop. So the question always shaking you now and then, do fish sleep at all? If yes, then how do they sleep? So are you ready to know? Here we go…

Before we answer do fish sleep, we should know about –

What is sleep?

The fact the dictionary describes the sleep is that sleep means animal requires to have their eyes closed, and part of the brain (the neo-cortex) must be shut down. In simple words, below criteria are generally considered as a sleeping state of any animals.

  • A specific sleep posture (e.g. lying down for humans, or sleeping in a cave or at the bottom of the seabody for certain species of fish.)
  • Inactivity or slowing down the metabolic rate.
  • The state is reversible (they can be woken up again)

But before we compare with humans about sleeping, we should know these things –

do fish sleep
Do fish sleep? What’s the difference between us and them?

The difference in sleeping between humans and fishes

Firstly, Fish do not have eyelids so they don’t need to sleep closing their eyes like us. Why? Because we have eyelids to protect our eyes from dust and other particles in the atmosphere. While underwater rarely happens to face dust particles by fishes. [Though exceptions are always there e.g Sharks. But still, there is a lot of difference between them and humans eyelids.]

Besides, they don’t have a neocortex. In humans, the neocortex is a part of the brain processes complicated tasks. As fish lacking neocortex and don’t have any complicated tasks, they don’t need that rest.

However, just because fish don’t have the same anatomy as mammals, doesn’t mean they don’t need rest or sleep. Some species become so deeply asleep that they can be lifted all the way out of the water without waking up at all. It’s just a different type of ‘sleep’ to the kind we describe as humans.

Why do we as well as fish Sleep
Why do we as well as fish Sleep?

Why do we as well as fish Sleep?

We know the very basics of why we need sleep and that is to give bodies the chance to rest and repair.

In human sleep, our eyes close, muscles relax, and we go through different stages of sleep; slow-wave sleep (deep restful sleep) and REM sleep (rapid eye movement sleep in which we sometimes remember our dreams).

It appears that the reasons humans sleep are much more complex to the reasons a fish sleeps. But the simple reason is that fish sleep to rest their bodies.

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Do fish have sleep cycles and how do they know when to go to bed?

Yes, most fishes have cycles of sleeping which is generally seen at night. Interestingly, if fish are exposed to too much light and are kept awake for long periods, they’ll sleep longer the next day like humans.

A study from Stanford University School of Medicine suggests, similar to humans, fish produce hormones that control their sleep patterns and internal body clocks. That is to say, fishes living in dark caves, even some evolved to have no eyes – can sleep in a regular daily pattern.

Like people, fish have an internal clock that tells them when to do things like sleep and eat. So even if you accidentally leave the lights on at night in the aquarium, the fish go to sleep anyway.

Thus, they can easily know when to go to bed! Have a Good Night, Dear Fish!! 😉 

How do they sleep? Their sleeping style?

Why do fish sleep with their eyes open?

It’s kind of creepy watching a creature sleep with its eyes open. But lacking eyelids, fish don’t have any choice opening eyes while they’re sleeping. Therefore, there’s no way for them to close their eyes.

Can fish sleep upside-down like humans?

In general, it’s not normal for them to lie upside down, and if you see this happening, it’s an indicator that something is wrong with them or you may need to do some investigating.

With that said, yes some of them sleep on their side as we sleep on our bed. Species like the Clown Loach have been known to sleep upside-down. Though it’s weird, some newbie fish lovers get lots of panic looking at their strange way of sleeping in the aquarium. Funny to say, some really think that fish either died or have issues but in reality, you know!

How to know whether the fish is sleeping or not?

Do fishes really sleep? – The answer is “YES”, almost all fishes do. Whilst it’s very easy to spot when most mammalian creatures are sleeping, it is a little more difficult at first sight to spot whether fish are sleeping. However, the way of their sleeping differs from fish to fish. Still, looking at some of the signs below may help you to decide if they are sleeping:

  • Inactivity for an extended period of time.
  • A resting posture. (Like staying into a cave or stone, staying clam at the bottom of the water body)
  • A routine of sleeping at a similar time, in the same way, every day.
  • Decreased sensitivity to external objects or stimulus.

[If you have an aquarium, you can closely observe their behaviour]

In an aquarium, you’ll be able to tell when a fish is sleeping because it will have either slowed down or stopped moving altogether.

One funny thing here to give you some of us like me during sleeping can’t even experience Magnitude 10 earthquake 😆 😆 Where the majority of fishes stay alert to danger even in their sleeping, so they can make a quick escape if they need to.

No Sleep Moment
What will happen if we and most fish don’t take sleep? – No Sleep Moment!

What will happen if we and most fish don’t take sleep?

Where we humans don’t have enough sleep, it starts to impact our ability to function properly, a similar thing happened in fish also.

In a 2007 study, a group of zebrafish were sleep-deprived during their normal 6 hour period of rest. Next day, the zebrafish were much harder to arouse, their mouth and gill movements were reduced. Thus, it’s proved sleep is very important for refreshing the mind and body. Without it, not only fish but also humans may be less aware of what is going on around. So, if you’re tired, take rest! 😀

Some fishes need no sleep at all!

Some types of species don’t need to get any rest. These include blind, cave-dwelling fish and deep-water species who swim continuously.

But how they still live and why they don’t need sleep? That is said sleep is the maintenance of our bodies when it goes through processing information and visual stimuli from the day. Both cave-dwellers, who have no sight, and deep swimmers have little to no information to process, which could deny the need for them to rest.

Do fishes see dreams as we do?

It will be a wonder if fish dream while they are sleeping. So far there is no proof of it but recent video footage of a sleeping octopus showed it changing colours, suggests it may have been dreaming about hiding from a predator or sneaking up on its own prey. 

Do fishes have to face Insomnia as we do?

Not all fishes, but a few! Thus, they can suffer from sleep disorders or sleep deprivation or Insomnia just like humans. For instance,  Zebrafish, which naturally drop their tails low and stay at the bottom of tanks or just beneath the surface of the water to sleep. When they are lacking hypocretin receptors, can display signs of insomnia (having trouble floating in the water and not sleeping for as long once they do).

Types of sleeping in fishes!

As we said earlier, fish still sleep. But some sleep during the day and only wake up at night, while others sleep does the reverse.

So, do fish sleep? The answer is …………………………….. it varies from fish to fish. 😯  😯 Let’s see some example of types of sleeping in fishes:


The common type of sleeping is a reduced rate of movement and a slower heartbeat. Both of these signs indicate that the body of fish’s metabolic rate has reduced and that they are conserving energy.


Most surface fish are diurnal, meaning they are active during the day and sleep at night. So, mostly rely on their eyesight to keep them safe from predators and find food. Though their sense of smell is good, water currents can interrupt this sense and make it less reliable.

Therefore, it makes sense that they are active during the day when they can see food easily, and sleep at night when their eyesight isn’t effective.


Some fishes like to live near a coral reef or between rocks or corals to keep themselves hidden and out of the sight of predators. Brown bullhead catfish is a good example of it. Though some others just drift with the occasional flick of a fin to keep them steady.

To avoid being swept away by the ocean current, a number of coral fish, such as some Damselfish, stuck themselves between reef branches. But to breathe they have to switch on their fins up to twice the normal speed to generate a flow of oxygenated water across their gills.


Some species of fish stop sleeping at certain periods in their lives, such as when they are caring for their young, or when they are migrating.

Species such as the stickleback make the sacrifice of parenthood to salute.  To stop their eggs from suffocating, the male has to constantly fan oxygenated water over them – often for several days or weeks. What’s the sacrifice? It’s turned very common seen for new fish fathers to die from exhaustion for not to sleep!


Some fishes have a weird style of sleep. For instance, Parrotfish has a very interesting sleeping ability. It secretes mucus which surrounds its body providing it with a cocoon-type outer layer. This protects them from predators while they are resting.

It not only means prey will find it harder to smell them but this sticky sleeping bag which protects them from parasites attacking them during their sleep.


There are even some fish that appear not to sleep at all such as tuna and some sharks. They need to keep constantly moving in order to allow water to keep passing over their gills to extract oxygen. It’s likely that these fish sleep with half their brain at a time, just like dolphins.

Fishes like the mackerel and the bluefish also show fewer signs of sleep. Even though they are less active during the night, they remain responsive to external stimuli and swim constantly.


Some like to sleep at the bottom of the seabed or aquarium. If you’ve ever owned a goldfish or watched one up close, you’ve probably noticed the times when it’s sleeping. It hovers near the bottom of the tank. If you put food in the tank during this time, you’ve probably noticed that it takes longer for the goldfish to respond or even not respond at all.

They usually sleep when the lights of the aquarium are turned off, being diurnal, they are active during the day and sleep at night.

Guppies, beautiful betta fish are also some example of this. They like to find a secure place to rest, usually among some leaves or structures, tank ornaments, and even behind a filter. If you own a betta, make sure you provide plenty of hiding and resting spaces for them!


Whereas some fish don’t even begin sleeping until they’ve reached adulthood, for example, Tilapia don’t start showing any signs of sleeping until around 22 weeks old.


Remarkably, some can easily switch their sleep pattern if the needed. For instance, white sucker fish can become nocturnal when separated from “Swarms of fish”. Why? as it’s too dangerous to swim by day alone, they’re much less likely to be hunted by predators at night.

Try not to disturb your fish while they are resting as this can frighten them which can lead to stress.

They can adjust their sleep time and rhythm

Unlike humans, they do not seem to follow a circadian rhythm as strictly as we do. They’ll easily adjust their sleeping patterns to water temperature, food availability, migration patterns, and even whether they’re new parents taking care of babies. So, we can potentially take that last part back because new human parents tend to miss out on rest, too!

Hope you enjoyed this post. Thanks for reading! See you soon 😉

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