How Do Fishes Sleep in an Aquarium?

-- By Carol

Here’s an interesting fact about fish.

Fish don’t have eyelids. They can’t close their eyes even if they wanted to.

So the question is, do fish sleep? Yes, they do.

Most animals on land, mammals, birds, and reptiles, sleep. Its been scientifically proven that sleep enables bodies to:

Recuperate by repairing worn-out tissues.
Process new information and memories gathered from stimuli.
Boost the immune system.

For wild animals, sleep also helps them to:

Hide from predators.
Avoid competition for food and water by alternating day and night times.

Therefore, it’s clear sleep is a critical part of animal lives.

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How Do Fishes Sleep in an Aquarium?

Thus, fish need to sleep too. It’s essential for their healthy growth and prolonged lifespan.

But how do they do it?

Find a Spot to Sleep

When you want to sleep, the first thing you do is decide where.

Do you sleep on the couch, bed, or the floor?

Fish follow a similar process. They spot a safe place to sleep first before hitting slumberland.

In an aquarium, you’ll notice the fish moving to the bottom of the tank or trying to hide behind plant growth.

Also, if there are solid objects such as rocks in the fish tank, you might see the fish moving to lie on them.

Other fish move near the surface of the water.

In the sea, most fish swim towards a coral reef or a cave. Coral reefs and caves are sheltered from light and also act as safe hiding places from predators.

Stop Swimming

Although fish can’t close their eyes to sleep, most induce sleep by slowing down their movements.

This includes slowing down their breathing and also gill movements.

They pause all activity and lie motionless for long periods.

Some, like the parrotfish, make themselves a sleeping bag of mucus where they lay motionless and sleep. The mucus protects them from parasites.

However, not all fish can stop being active and lay motionless to sleep.

Tuna fish and some species of sharks have to continue swimming while sleeping. It’s necessary to keep swimming to facilitate breathing by allowing oxygen-rich water to flow over their gills.

To allow maximum rest, sharks have to slow their swimming speed.

Stop Responding to Gentle Stimuli

When fish are asleep, their awareness of what’s happening around them is reduced.

While humans have a neocortex that shuts down when they sleep, fish never completely shut down even though there’s evidence of low brain activity when asleep.

This is because, one, they don’t have a neocortex, and two, they need to keep alert of danger like predators approaching.

However, due to slowed breathing and brain activity, sleeping fish have little or no response to gentle movements.

You can put your finger inside the tank, and the sleeping fish will be none the wiser.

Create a Sleeping Habit

One of the most popular habits among human beings is sleeping at night.

During the daytime, humans are up and about. When it’s nighttime, they take a rest.

Similar to humans, fish have their sleeping habits.

Most are diurnal, which means they’re active during the daytime and rest at night.

The human-like sleeping pattern can be explained by the fact that their eyesight is better in the daytime. In the case of an aquarium, the eyesight is best until the light goes off.

However, nocturnal fish do exist. A few of the catfish and knife fish species are active at night.

During the night, they look for food, play, and explore the waters. Then it’s daytime; they search for crevices and caves to hide in and sleep.

Are There Fish That Don’t Sleep?

So far, we’ve covered two types of fish, diurnal fish and nocturnal fish.

Diurnal fish sleep like humans. They’re awake and active during the day and choose to rest at night.

On the other hand, nocturnal fish sleep during the day after being active all night.

Now we come to the third type of fish, those that don’t sleep.

In this third category, there are three types of non-sleeping fish:

Fish that don’t sleep at all
Fish that naturally don’t sleep for certain periods
Fish that are experiencing insomnia

Fish That Don’t Sleep at All

It’s not possible to have aquarium fish that never sleep at all.

Fish sleeping is influenced by light changes in the aquarium.

Unfortunately for fish dwelling in caves, darkness rules supreme.

Fish such as blind fish don’t sleep at all because the caves are ever dark.

They can’t tell between daytime and nighttime. Thus, they’re incapable of sleep.

Fish That Naturally Don’t Sleep for Certain Periods

The fish here are capable of sleeping.

But for specific periods, they can’t sleep.

For instance, when the fish are caring for their young, they don’t sleep.

Another example is when fish are migrating to tropical or cold waters for breeding or food. This only happens with wild fish, not aquarium fish.

Also, some species can’t sleep when they’re young. A case in point is tilapia fish. They don’t sleep until they’re six months old.

Fish That Are Experiencing Insomnia

By definition, insomnia is habitual sleeplessness.

Is it possible for fish to suffer from the inability to sleep consistently?

In humans, insomnia is often caused by poor sleeping habits.

Therefore, scientists have taken it upon themselves to figure out if fish can experience insomnia by experimenting on the zebrafish.

The zebrafish is placed in a fish tank and subjected to constant bright light and activity. The increase in stress reduces the zebrafish’s ability to sleep.

When the zebrafish is returned to normal light and dark cycles, it sleeps more than usual but slowly re-adapts to normal sleeping times.

Thus, scientists believe that fish do experience sleeping disorders when exposed to unhealthy stress, such as harsh brightness

How Long Do Fish Sleep?

The estimated fish sleeping time is between eight and twelve hours.

It’s difficult to approximate the optimum sleeping period for fish.

But since aquarium fish experience night and day at almost the same time as humans, scientists believe they sleep as long as most humans.

Also read: Can Aquarium Fish See At Night/Dark?

Can Fish Sleep Facing Sideways, Upwards, or Upside Down?

Most fish sleep with their underbelly facing downwards.

During this time, they use their gills for balance by making small gill movements.

When aquarium fish are sleeping on their side or facing upside down, it indicates they’re unable to balance, which means there’s something wrong.

If it’s a goldfish, there’s a possibility of a swim bladder disorder. If the disorder isn’t treated immediately, the goldfish could die.

Other reasons why the aquarium fish are sleeping sideways or upside down include:

Poor water quality – Build up of toxins is making it hard for fish to breath
Overfeeding of fish – Overfeeding leads to constipation causing the fish imbalance
Inconsistent water temperature – Most fish are sensitive to significant changes in temperature.

How to Make Aquarium Fish Get Enough Sleep

When animals are deprived of sleep, their health deteriorates. Most importantly, their mental cognition lowers, and their minds are less sharp to the environment’s stimuli.

Therefore, it’s crucial to ensure your fish are getting adequate sleep.

For starters, regulate light exposure in the aquarium to introduce the fish to consistent day and night cycles.

At night, you can do this by switching off the aquarium light at regular times to help the fish sleep.

If your aquarium fish are nocturnal, close the curtains to make it dark and assist the fish to get some rest. You can also place a blanket on the aquarium.

Secondly, increase plant cover in the fish tank. There are different kinds of live aquatic plants to put in fish tanks, such as:

Anacharis
Duckweed
Amazon sword
Java fern

Increasing plant cover creates more hiding spots for fish. The idea is to improve a sense of safety for the aquarium fish while they’re asleep.

Lastly, change the aquarium water regularly. Decomposition of fish feces and leftovers releases ammonia and nitrates into the water. Also, dissolved oxygen is used up while more carbon dioxide is produced.

The buildup of toxic gases in the water causes the fish stress and breathing difficulties. Thus they’re robbed of good sleep.

Want to Learn More About Aquarium Fish?

Fish are mysterious creatures. They invoke a sense of wonder and appreciation.

Indeed, you can never know too much about them.

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