Do Aquarium Fish Sleep? [How, How Long, And Where They Sleep]

With fish not having eyelids, it’s pretty tough to know whether or not they’re sleeping, or if they even sleep at all. So, do aquarium fish sleep?

Yes, the majority of aquarium fish do sleep. Most of the fish sleep while being almost motionless. Other fish, like tuna and some sharks, need to keep moving while they sleep so that they can breathe. However, blind cavefish don’t sleep. They rest whenever required and don’t fall asleep entirely.

Let’s now see how fish actually sleep.

How Do Fish Sleep?

Many aquarium fish sleep with the front of their bodies slightly tilted upwards. This is normal behavior.

Some species of fish only start sleeping after they reach the age of 5 or 6 months.

Many fish also give up sleeping to take care of their young.

Fish are diurnal (active during the day) and prefer to sleep mostly at night in the darkness, just like humans.

Also, most fish don’t sleep while they’re migrating.

The Wrasses and Parrotfish wrap themselves in mucus cocoons before sleeping.

They do this for protection against parasites and predators.

Moreover, most fish sleep very lightly and can easily be woken up if the need arises.

How Can You Tell If A Fish Is Asleep?

There are a few telltale signs to determine whether or not your fish is sleeping.

Given below are a few of the biggest indicators that a fish is sleeping:

Stopped Moving Around: Most fish, especially aquarium fish, float in the water and don’t swim around while they are asleep. Sometimes they’ll flap a fin now and then to stay balanced while sleeping, but they won’t move across the tank. Even though the fish may look completely motionless while sleeping, they might move from one place to another so slowly that you won’t even notice.
Reduced Gill And Fin Movement: The gills and fins of the fish will move far less than when it’s awake.
Become Unresponsive: If your fish aren’t swimming up to the glass and checking things out the way they normally do, they’re probably sleeping.
Resting At The Bottom Of The Tank: Sometimes, you might see your fish resting its body at the bottom of the tank or a decorative item. They remain very still in this sleeping position and only flap a fin occasionally to maintain balance.

How Long Do Aquarium Fish Sleep?

Aquarium fish usually sleep for around 10 hours per day.

This mostly happens while you’re sleeping, and the lights are off.

However, depending on the type of fish you have, this time could be a little higher or lower.

Fish also take multiple power naps throughout the day without you noticing, but most of their sleep will be in the darkness.

Do Fish Need Complete Darkness To Sleep?

Fish can sleep even if you leave the light on all night. However, it isn’t healthy for them to live like that every day.

Just like humans, fish need a certain amount of darkness every day to maintain good health.

However, some fish are nocturnal (active at night).

When the lights are off, they will use this time to explore the aquarium and move around.

They’ll mostly hide in caves and other decorations during the day.

Do Fish Sleep At The Bottom Of The Tank?

Many fish do sleep at the bottom of the tank.

However, if you notice your fish hanging around the bottom of the tank more often than normal, it may be a sign of sickness.

Do Fish Sleep At The Top Of The Tank?

Fish normally don’t sleep at the top of the tank. They could be doing this because there isn’t enough oxygen in the water.

The oxygen levels at the top of the water are higher. So they might be trying to get the amount of oxygen they need up there.

Other warning signs of low oxygen include:

Slower swimming and movement in general.
Increased gill movement.
Struggling gasps for air.

If your fish are spending all their time near the surface of the water, you need to urgently increase the oxygen levels.

Do Fish Sleep Upside Down Or Sideways?

Fish do not sleep upside down or sideways.

The only healthy position for most fish to sleep in is the straight upright and/or slightly tilted up position in the front.

If you notice your fish swimming or sleeping sideways or upside down, it may be suffering from a disease of the swim bladder.

This is the organ that helps a fish with buoyancy.

However, there are other illnesses that can also cause a fish to be lopsided, like a bacterial infection.

Either way, you’ll have to have a veterinarian check out your fish if you notice this type of positioning.

Do Freshwater Aquarium Fish Sleep?

Freshwater aquarium fish do sleep.

You might notice them sleeping at the bottom of the tank or around your aquarium decorations.

They take naps during the day and sleep mostly at night.

What Happens To A Fish When It Sleeps?

Given below are some important things that happen when a fish sleeps:

Most fish become almost motionless in the water. They keep hovering in one place. However, a few types of fish swim while sleeping.
As a fish sleeps, its metabolism slows down to conserve energy. This recharges its body.
Fish don’t experience the REM state of sleeping as most other animals do.
While it relaxes, it subconsciously keeps track of the environment around it and can easily wake up.

How Do You Know If A Fish Is Sleeping Or Dead?

Here are some of the things to look out for to know if a fish is sleeping or dead:

Dead fish float on the surface of the water.
Sometimes dead fish also lay sideways on the bottom of the tank.
Dead fish don’t have any gill or fin movement. Even the gills and fins of a sleeping fish move occasionally.
Grey or sunken in eyes are signs of a dead fish as well. This happens a few hours after death.
If the fish isn’t moving and its scales have changed color overnight, it’s likely that the fish is dead. Sometimes the scales may get damaged by other fish eating it.

If you’re not sure, touch the fish with your fishnet to see if it moves. If it doesn’t, it’s dead.

A dead fish must be removed from the water immediately, or it may spread diseases to the other fish.

Related Further Reading:

What If You Leave The Fish Tank Light On 24/7?

Healthy fish need to experience the diurnal-cycle; this is the cycle of day and night.

Fish might not die if you expose them to light 24/7, but they will live longer and healthier lives if you give them 8 to 12 hours of darkness.

If you want your tank to be lit all night, use LED lights that simulate night time.

These are called Moonlight LEDs. They are blue LEDs and are as soft and gentle as the moonlight.

Can Fish Sleep With A Red Light On?

Red light is reportedly better than blue light for fish that are trying to get some quality sleep.

This is because it’s believed that fish can’t see the red light at all.

If you only have regular lights to illuminate your tank, attach a timer to the plug.

This way, you won’t have to worry about forgetting to switch off the lights at night and turn them on in the morning.

Can You Turn The Filter Off At Night?

You should never turn off the filter, so make sure it isn’t on a timer plug with the light.

The filter not only cleans the water but also aerates it for your fish to have enough oxygen.

The filter is also essential to keep the water well circulated.

Interesting Further Reading:

Parting Thoughts

While most fish do sleep, they do so very differently from mammals.

Of course, they can’t close their eyes or lay down on their sides to have a snooze.

Instead, they sleep in a static floating position and sometimes rest against the bottom or on the tank’s decorations.

Fish are very light sleepers that can easily be woken up.

Some fish swim while they are asleep, like the tuna and the sharks.

They do this because they need to keep the water flowing over their gills. Very few fish don’t sleep at all, like the blind cavefish.