How Do Fish Sleep? Read this article and find out!

How do fish sleep? We humans know that our bodies need sleep for optimal health and well-being. But how does it work with fish?

Sleep is not only essential for restoring and rejuvenating our bodies, but also for solidifying and consolidating information.

It is also easy to determine when humans are sleeping, however with fish this is a little more difficult.

In this article, we’ll look at whether fish actually sleep, how to determine if they are sleeping, the way different species of fish sleep, and how often fish sleep.

Do fish sleep?

Humans and fish sleep very differently

The simple fact that the popular definition of sleep requires the animal to have its eyes closed and part of the brain (the neocortex) turned off makes it difficult to answer the question ‘Do fish sleep?’ with a yes.

Fish do not have eyelids or neocortex. However, it is also important to observe their behavior and not just their anatomy.

Just because fish don’t have the same anatomy as mammals doesn’t mean they don’t need rest or that they don’t sleep. It’s just a different kind of ‘sleep’ to the kind we experience as humans.

One study carried out by the University of Zurich evaluated and studied sleep in nearly 200 species of animals, including fish. He found that while the way animals sleep varies widely, all animals sleep in some specific way.

While fish might not fit the exact dictionary definition of what sleep is for us – they still have periods of inactivity and sluggishness, which is their version of sleep.

Yes, fish sleep!

What is observed in fish sleep is a reduced rate of movement and a slower heartbeat. Both signs indicate that their metabolic rate has slowed and that they are saving energy.

In the study mentioned above, sleep was defined using the definition that was first stated by a French psychologist in 1913 . The definition of sleep consisted of the following criteria:

  • Species-specific sleep posture (e.g., lying down for humans or sleeping in a cave for certain species of fish).
  • Maintaining behavioral stillness (inactivity or slowness)
  • They are hard to wake up
  • The state is reversible (they can be awakened again)

Using this definition, it is much easier to determine that yes, fish do sleep..

How do fish sleep?

While it is very easy to identify when most mammalian creatures are sleeping, it is a little more difficult, at first glance, to identify whether fish are sleeping.

Fish don’t close their eyes – they don’t have eyelids and they also don’t show signs of sleep REM (rapid eye movement).

Instead, the fish become inactive; how inactive they become depends on the fish species. Most fish hover or stand still and their gill movements slow down.

Most fish remain alert to danger, even while sleeping, so they can quickly escape if necessary.

In an aquarium, you will be able to tell when a fish is sleeping because it has slowed down or stopped moving altogether, entering a floating state.

Why do fish sleep?

Sleep research is still in its infancy, and much of it remains unknown.

We know the basics of why we need sleep, and that’s giving bodies a chance to rest and repair.

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

It appears that the reasons humans sleep are much more complex than fish sleep, although research in this area isn’t extensive enough to say definitively.

The most likely and simplest reason fish sleep is to rest their bodies.

When we humans don’t get enough sleep it starts to impact our ability to function properly, something similar has been observed in fish.

In a 2007 study , a group of zebrafish were sleep deprived during their normal 6-hour rest period; the next day, the zebrafish were much more difficult to awaken and their mouth and gill movements were reduced. The study also showed that they have slower breathing cycles during sleep, take naps and are governed by melatonin – very similar to us as humans.

When do fish sleep?

Most surface fish are diurnal, meaning they are active during the day and sleep at night. Due to their eyesight, it makes sense that they would be active during the day when they can see food easily and sleep at night when their eyesight is not effective.

When fish sleep really depends on the environment and the type of species they are. For example, fish that live near coral reefs tend to stay active during the day and rest at night to avoid predators.

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

While some fish don’t even begin to sleep until they reach adulthood, for example, the Tilapia does not begin to show signs of sleep until 22 weeks of age.

Most aquarium fish sleep when the lights are turned off overnight.

Do all fish sleep the same way?

The way a fish enters a “dream state” varies depending on its species, where it lives and its activity level.

Some fish nest in the substrate; others hide in caves or among corals, while others just float occasionally with a flipper to keep them steady.

Parrotfish have a very interesting sleeping ability. It secretes mucus that surrounds the body, providing a cocoon-like outer layer. This protects them from predators while they are resting.

Some fish, like certain species of sharks, need to constantly move to allow water to continue passing through their gills to extract oxygen.

Other fish wedge between rocks or corals to keep themselves hidden and out of the sight of predators.

There are even some fish that don’t seem to sleep, like tuna for example.

While research on sleep in fish is still in its infancy, it is believed that the reason some fish seem not to sleep may be due to the fact that their landscape doesn’t change that much and therefore they don’t have much to process or consolidate. in memories.

Watching large schools of fish sleep in the wild is also interesting. Part of the group will act as eyes for the whole group while some fish rest and then molt.

How to know if fish are sleeping

The top four signs that fish are sleeping include:

  1. Inactivity for a long period of time.
  2. A resting posture.
  3. A routine (sleeping at a similar time, in the same way, every day).
  4. Decreased sensitivity to external stimuli.

You will be able to tell if your fish aquarium are sleeping using the above criteria.

They may be a little adrift, but they will seem almost inert. Try not to disturb your fish while they are resting as this can scare them, which can lead to stress.

Guppies prefer to sleep in the dark and sleep at night when all lights are off. You will be able to detect if your guppy is sleeping if it is hovering in one place (usually just above the substrate). Sometimes they float on the surface – remember that if this happens during the day, they are more likely to not sleep and this indicates a more serious problem.


While it is clear that fish sleep differently than most mammals, it is evident that they require periods of rest in which their bodies slow down, and it is undeniable that fish enter a sleep-like state.

From the many examples we’ve provided in this article, you should now see that the way different species of fish sleep differs dramatically.

From continuing to swim while sleeping, to floating close to substrate, to hiding in caves or corals – fish have many different ways of sleeping, but most require time off to allow their bodies to rest.