How Do Fish Sleep? I Was Surprised Too

There is ongoing research being done on how fish sleep, so there is still a lot we don’t know. While fish don’t sleep in the way that we humans do, most fish do have periods of rest.

Different fish sleep or rest in different ways – some cozy up at the bottom of the water, others hover close to the bottom, and some sleep so deeply that they can be lifted out of the water without even waking up. Some even keep swimming while they are resting.

While most fish do need to get sleep at night, their sleep mostly resembles a sort of daydream or trance state.

Fish have to be alert for danger during their sleep, so they reduce their metabolism and activity but are still watchful of their surroundings.

Fish typically go into a sort of period of suspended animation rather than full on sleep, but this might have the same restorative benefits as sleep does for us.

Do All Fish Sleep?

Almost all fish do need sleep, but some species don’t. Blind cave-dwellers and deep-water fish swim continuously, and neither of them need to sleep.

Sleep is how we all process information and visual stimuli from the day. Cave dwellers and deep-water fish either have no sight or no information to process. This could be why they don’t need to sleep.

Fish that swim continuously like tuna and some sharks don’t seem to sleep. Researchers are still trying to work out why that is. They think it is because these fish are constantly swimming in deep ocean water, where there isn’t much change in scenery, similar to cave dwellers and deep-sea fish. Since they aren’t getting much information and visual stimuli to process, they too may not need to sleep.

Some fish can go a long time without needing to sleep. For example, during migrations, spawning periods, and when raising their young, fish often won’t sleep at all. Some fish don’t even sleep until adulthood.

A study on the Mozambique Tilapia found that they don’t sleep at all for the first 22 weeks of their lives.

How do fish sleep?

Fish sleep in all different ways depending on the species:

  • Drifting

Some fish simply drift with an occasional flicker of their tail or fin to keep them steady. They have very limited movements compared to when they’re swimming. Often the fish that are kept in aquariums sleep this way.

  • Burrowing

Some fish sleep in holes or under rocks and others create nests in the sand and sleep there. In coral reefs, some fish will sleep in the coral but keep their bodies moving as though they are swimming. The fish’s movements increase oxygen flow to the coral in return it helps to shelter the fish.

  • Sleeping in Schools

Some fish sleep in schools so that while they sleep the others can look out for predators.

  • Laying Down

Some fish will even sink to the bottom so that they can relax and get into a sleep-like state. An example of a fish that sleeps in this way is the Woolfish which can look like it is playing dead.

  • Other Ways

Some types of parrotfish release mucus from glands in their gills which forms a cocoon around them. They sleep inside this cocoon as it protects them from parasites and masks their scent from predators.

The reef dwelling Spanish Hogfish sleeps so heavily that it can be lifted up out of the water before it wakes up.

Fish have to sleep in a way that enables them to be alert for danger even while being asleep.

When Do Fish Sleep?

Scientists believe that most fish are similar to humans and other animals, in that they have regular sleep schedules.

Most aquarium fish are diurnal, so they rest at night and are active in the daytime. Some species however are nocturnal and spend the daytimes sleeping in a cave or somewhere dark.

Unlike humans, many fish are able to switch back and forth from daytime to nighttime sleep depending on their surroundings. They can change their sleep patterns depending on the temperature of the water, food availability and the presence of predators.

Atlantic salmon, for example, become nocturnal in cold waters. This switch in their sleep pattern may be a safety precaution so that they can use the darkness of night time to protect against predators.

Why Do Fish Sleep?

Similar to humans, sleep time for fish is when their bodies and brains are rested and restored. Fish brains aren’t nearly as complex as human brains so they don’t go through the entire sleep cycle that we do. They don’t experience REM sleep for example.

Sleep promotes health in fish, just like it does in humans. If they don’t get enough sleep, after a while they can become exhausted and more succeptable to diseases and illnesses.

How Can You Tell If a Fish Is Sleeping?

Since fish don’t have eyelids, except for some shark species, they sleep with their eyes open. This is why it can be difficult to know when a fish is asleep. They also might be sleeping while still moving, or even swimming around.

There are some behaviors that you can look out for which suggest your fish is sleeping:

  1. Staying motionless for extended periods of time. There may be subtle fin and tail movements.
  2. Less sensitivity to what is going on in their surroundings.
  3. Floating in a resting position or laying at the bottom of their environment, maybe in a cave or under some driftwood.
  4. They may be in a routine and sleep at the same time every day – usually when it is dark.

If you do observe your fish sleeping, try not to disturb them as it can lead to them becoming stressed.

How Do Large Marine Animals Sleep?

So, we know that smaller fish have found a number of ways to squeeze periods of rest into their days, but how do the bigger fish do it? Here are a few examples:

  • Whales

Even the biggest marine animals in the ocean find time to catch up on their sleep. They are able to sleep while still swimming. Whales can actually shut down half of their brains while the other half keeps the whale breathing and moving at the water surface. Mothers will guide their young while they sleep next to them.

  • Sharks

Most sharks belong to a fish species that have to swim in order to keep breathing. Because of this, sharks can’t stop to sleep. It is possible that sharks don’t sleep at all. There is recent research however that suggests they may be able to shut down part of their brain, which allows them continue swimming. This would enable them to rest in phases.

  • Dolphins

Researchers have found that dolphins sleep for short periods of time while floating just below the surface of the water. Dolphins’ brains are always alert, even just partially.

  • Octopuses

Octopuses don’t have the same sleeping needs that humans do because they are invertebrates. When they do sleep, they will often crawl into their homes and rest with their eyes narrowed. We don’t know if they are actually asleep, but their brain waves during this resting period are similar to ours at the REM stage of sleep.

Can Fish Get Insomnia?

The only fish that has shown signs of insomnia is the Zebrafish. It is easy to tell when this fish is asleep because it shows clear signs. It will stop moving and be significantly slower to respond to external stimuli. It’s breathing and heart rate will also slow down.

Scientists researched the zebrafish extensively to see how they would respond to sleep deprivation. They stimulated the fish to prevent them from sleeping and noticed that they showed a rebound effect. Once the fish were left alone in a dark environment they slept for a longer period of time to make up for the sleep they had lost.

This only worked when the lights were off, as the fish wouldn’t sleep if it was light. Because of that, scientists concluded that sleep deprivation didn’t take the same toll on the fish as it does with humans.

How to Ensure Your Fish Can Get Enough Sleep

The best way to ensure that fish in your aquarium are sleeping enough is to put a light in the tank. If you have it on a timer that will make their day and night routine consistent.

It is important that the tank gets dark enough so that your fish can get sufficient sleep. Try to limit the amount of light they get during the night.

It is also a good idea to provide plenty of shelter where your fish can feel safe to rest.  Some good decorations to add for this are caves and plants.

Conclusion

Fish sleep cycles are a lot more complex than one might think. Because most fish are always moving and being alert to danger, they developed clever ways to be able to rest and recharge their bodies while still being aware of their surroundings.

In a tank environment, it is important to make your fish comfortable so that they can sleep. Fish, just like humans, need sleep in order to be healthy.

Frequently Asked Questions

Do Fish Dream?

No, fish have no neocortex in the brain so they don’t dream.

How Long Do Fish Sleep For?

Fish will sleep for up to about 12 hours depending on how dark it is in their environment. They typically need about 9 to 10 hours. This is why it is important, if you keep fish an aquarium, to provide them with a good amount of darkness and a consistent day/night schedule.

Will Tapping on The Glass Disturb My Fish?

Yes! They have a remarkable sense of hearing and are highly sensitive to vibrations. Also, sound is amplified by a magnitude of five in the water, so it is extremely disturbing for them.

Will My Fish Sleep If I Leave the Lights On?

If you leave your aquarium lights on all the time, your fish will probably exhaust itself by continually swimming. They may then lie at the bottom of the tank for an extended period of time during daylight hours. This is why it is so important to have a clear day and night in your tank. Exhausted fish are more succeptable to diseases and illnesses.

Can Fish Sleep with Blue Light?

It can be fun to have blue light in your aquarium as It enables you to see your fish during their night time. To the fish it will be dark but we can still see them. Your fish will still be able to sleep with blue light, they aren’t affected by it.

Do Fish Have to Sleep at Night?

No, if you can make the tank dark enough during daylight hours then the fish will think it is nighttime and be able to get some sleep. Be sure that if you are going to do that, there are no lights on in the room and no blinds open that can disturb their sleep. Be sure that they get enough light in the tank during nighttime so that they are able to go about their business and any plants in the tank are able to grow.

How Do Betta Fish Sleep?

Betta fish take short naps during the day. If you notice your betta lying at the bottom of the tank, he is probably sleeping. They sleep in a variety of places and positions – some will squeeze into small spaces, others will hide in an ornament in the aquarium. They might also nestle themselves in the gravel of your tank, or even just float around at the top of the tank. If your betta fish is sleeping excessively, he might be bored or even sick.

Should You Put Your Fish Tank in Your Bedroom?

It can be a good idea to keep it in your room so that when you sleep and have your lights off, you don’t disturb your fish’s sleep. However, if you sleep with lights or TV on, it isn’t a good idea because it may disturb your fish. Also, filter and purification systems can be very loud which might disturb your sleep.

 

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