When Do Fish Sleep? The Sleeping Habits of Our Underwater Friends

September 15, 2020

When Do Fish Sleep? The Sleeping Habits of Our Underwater Friends

Fish look so peaceful when they swim through the water. Their movements are sanguine – seemingly moving without putting any effort into it. So when do fish sleep? Are there times when they are actually sleeping and we don’t notice? 

Here at Finn’s Fishing Tips, we’ve been around fish our entire lives. From salmon fishing at a young age to catching trophy yellowfin tuna, we love our underwater friends. So when do fish rest? Do fish actually sleep at all?

The answer is unclear! Research shows that fish do not sleep as land animals do. Rather, they lower their bodies’ metabolism and reduce their activity for extended periods of time to rest. During this time they might not move, but they are usually aware of their surroundings.

Since fish have to be aware of potential predators at all times, they have evolved to not really ever sleep in a traditional sense. When they do rest, they often find a safe location such as a rock tunnel or nest. This helps them avoid predators and properly rest.

How Do You Know When Fish are Sleeping?

As we mentioned above, fish don’t sleep in a traditional way – but there are still tell-tale signs they are “sleeping”. They’ll usually be sitting very still near the bottom of the body of water they live in. Their gills will be dilating in a very slow cadence, indicating their heart rate is especially low.

Since fish don’t have eyelids, you won’t be able to tell they are resting from their eyes closing! Look for fish that are particularly still. They’re most likely resting! You can also try and disturb them faintly to see if they react. If they are active, even the slightest disturbance will get a reaction. If they are sleeping, they’ll most likely ignore you.

You should also pay attention to how other fish act around them. If there are several fish sitting very still, chances are they are all sleeping. They might let small bubbles out of their mouths every once and a while as well.

Do Fish Need Darkness to Sleep?

For an animal that keeps its eyes open non-stop, you might think that they need darkness to sleep. This, however, is not the case. Fish do not need darkness to rest – even though they have adipose eyelids that are clear. Depending on the species of fish, they have different sleeping/resting rhythms than most land animals.

Some fish, such as tarpon and snook, are primarily nocturnal. This means that they spend most of their awake hours after it gets dark outside. This also means that they often sleep or rest during the daytime when it is bright outside.

Other species of fish, such as trout and bass, usually rest after dark. But all species of fish can rest or sleep pretty much whenever they choose to. Darkness is not a factor. 

Scientists discovered that some species of fish that live in caves and are blind actually never sleep. Since they can’t see anything and it is dark all the time for them, they remain relatively sedentary all the time. This means they hardly ever have to rest and never sleep.

How Many Hours Does a Fish Sleep?

The number of hours a fish rests depends on the species and the activity level of the fish. Some fish can be seen resting for only a few hours at a time, while others will sleep a whole 8 hours or more. It really just depends on the fish and it’s sleeping cycles.

The time of year it is may also affect how many hours a fish rests. When it’s the middle of summer and the water temperature is high, fish tend to be more active and sleep less. In the winter, when the water temperature is cooler, fish tend to rest for longer periods of time. 

Fish may also rest more often to keep their fat reserves high in the offseason. This will allow them to survive if they can’t find food for extended periods of time. Just because it is cold, however, does not mean all fish aren’t active. The winter season can be a great time to go fishing for certain species, such as musky or walleye.

Ice fishing is a popular sport in the great lakes region as well as other northern climates with cold water fish. If you want to go ice fishing, check out our guides on getting the proper ice fishing suit as well as how to ice fish for walleye.

Do Fish Swim While They are Sleeping?

As we mentioned above, fish most definitely swim while they rest. Most fish swim far less when they are resting, but you’ll often see them move their fins to stay stationary in the water. Some species, such as marlin and tuna, swim constantly – even when they are asleep. 

These species have to be continuously moving to find food and have evolved to rest while still swimming a decent clip. They are also some of the fastest fish in the ocean, so it isn’t surprising they continue to swim while they rest!

Fish Sleeping Habits

Fish sleeping habits are hard to track and often vary significantly species to species. As we mentioned earlier, some species spend most of the day asleep to hunt at night, while others sleep more regular schedules. Some types of fish only sleep a few hours every day, while others sleep all the time.

If really depends on the species of fish in question! Another factor that determines the sleeping habits of fish is where they are in their mating cycle. Fish that run (like salmon) or travel long distances to seek out a mate will often not sleep much during this time period. They have evolved to not need as much sleep while trying to find a mate.

Another factor that plays into fish resting habits is how old they are. Older fish tend to rest more frequently than younger fish (much like humans), while baby fish don’t sleep much at all. This is most likely because younger fish are small and have to be on high alert for potential predators, while older fish are large and don’t have to worry as much.

How to Help Fish Rest

If you are a fish owner, congrats! But how can you help your fish sleep? The answer is to try and make them as comfortable as possible. Follow the below pointers to help your fish rest.

  • Keep the water temperature at their natural habitats’ temperature. Depending on the species of fish, this could be fairly cold. Look it up and use a heater or cooler to regulate the temperature.
  • Feed your fish on a regular schedule. Look up an appropriate amount of food to be feeding your fish, and make sure you are feeding them that will keep them healthy.
  • Turn the lights off at natural times. When it gets dark outside, try and make sure the room your fish are in is also dark. This will best simulate the natural habitat of the fish.
  • Don’t introduce fish that don’t belong together. The last thing you want to do is stress your fish out by introducing a species they don’t get along with. This could be a natural predator of theirs or simply a species they normally would never be around.
  • Provide the fish with plenty of hiding places they can feel comfortable resting in. You can buy small rock caves and arches that will help fish feel like they are protected and at home.

Can Fish Suffer from Sleep Disorders?

It’s still unclear if fish can suffer from sleep disorders. Scientists have tried keeping fish awake with prolonged light, but the fish will often find a way to rest even under direct light. Since fish don’t sleep in a traditional manner, they can still get rest when it’s light out.

How to Observe Fish Sleeping

If you’re still curious about what sleeping or resting fish look like, there are several ways you can see them. The easiest way is to go to your local aquarium late in the day (or at night if it’s open). Look for fish that aren’t moving and are staying still. If the fish are near the bottom of the tank and aren’t moving, they’re most likely sleeping!

Another way you can see fish sleeping is to get an aquarium for your home. This will allow you to witness fish sleeping on your own time. It will also give you more opportunities to see fish sleep during strange hours, such as in the middle of the day or early in the morning. 

Getting an aquarium is much more expensive than heading to the public aquarium, however, if you’re really into fish it might be worth the investment! Plus having pet fish is fun.

If you’re lucky, you might even be able to see fish sleeping in out in nature. Some species that are particularly not scared of humans, like catfish and carp, can be seen sleeping in shallow water. Again, look for fish that don’t seem to be moving much and are stationary. They’re most likely asleep!

Parting Thoughts

The answer to when fish sleep isn’t straightforward. Most experts can’t even agree if fish actually sleep at all! But what they can agree on is that they enter resting periods and that their habits vary significantly with species. 

We hope you have learned more about when fish sleep as well as how to help your fish sleep better in their habitat. If you have additional questions you want to be answered about how fish sleep, feel free to shoot us a note in the comments below.

Happy Hunting!

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