Do Fish Sleep?

When you really think about it, practically all living organisms require some sort of rest in order for their bodies to function properly. Everything that consumes food, participates in social interaction, or anything that engages in physical activity will require sleep at some point. But what about fish?

You may wonder whether fish even sleep, since you don’t exactly see them laying down in a bed with pillows and a blanket sleeping all night long. Instead of that, fish seem to just swim around and engage in the external environment around them. But you’d be surprised to find out that fish actually do rest, and they sleep in ways that are different than our sleep.

What is Sleep?

Sleep is classified as the rest that our bodies take in order to gain energy to engage in activities throughout the day. More specifically, sleep is a phase in which our bodies as well as our brains rest, allowing them to refrain from engaging in their normal activities in order to gain the opportunity to function the next day.

At more of a natural level, our bodies follow a pattern that we call the circadian rhythm. The circadian rhythm that our bodies engage in is responsible for affecting the hormones in our bodies, which make us either feel energized or tired. When we sleep, we also go through several different stages, including the REM phase as well as the non-REM phase.

Scientists claim that humans need sleep in order to recover from the activities that we’ve engaged in throughout the day, and it even helps us muster up the energy that we will have the next day as well (1).

Benefits of Sleeping

Overall, sleep gives your body a wide range of needed benefits (2).

• Promotes a healthy heart
• Gives your body a chance to recuperate
• Reduces your risk of experiencing anxiety or depression
• May help you lose weight
• Increases your alertness throughout the day
• Improves your memory
• Reduces inflammation
• May prevent your risk of developing cancer

Why Fish Sleep

Fish have a specific resting mode that they engage in several times throughout the day. Since they are a living organism that are capable of moving around, seeing their external environment, and interacting with other organisms around them, they are able to use energy and therefore, they require rest.

Fish have a certain resting mode that they consider their sleep, so it’s not a deep REM sleep like we engage in. Similar to us, fish have a hormone known as melatonin that is responsible for regulating the sleep and wake cycle every day. Melatonin is produced by the pineal gland in humans as well as fish, and the pineal gland is only responsible for producing and releasing melatonin when it experiences dimmer lights, like nighttime.

Fish require some sort of sleep and rest mode in order for them to recuperate, heal any injuries that they’ve developed, and to fight off diseases, infections, or other types of illnesses. This resting mode requires little energy, so their bodies are able to then utilize that energy towards other processes in their body that need it. This resting mode of sleep also gives them a way to conserve more energy to operate for when they wake up.

Different Than Our Sleep

When humans sleep, we close our eyes, lay down, and a portion of our brain shuts down. When we engage in these activities, we are classified as sleeping, and we use this as a resting mode so that our bodies can recuperate. On the other hand, fish don’t close their eyes, since they don’t have eyelids. And part of their brain isn’t responsible for shutting off.

Even though they don’t engage in the same activities as humans when they sleep, they still have their own version of a resting mode that they consider sleeping. More specifically, fish’s sleeping is classified as periods of time in which they are inactive. In certain times of the day, fish have reduced movement rates and their heart rate even slows down, which gives their body a chance to conserve energy ad recuperate.

Overall, when fish are sleeping, they stay in a certain specific posture, they don’t engage in rigorous activity, they are difficult to arouse, and they can be woken up at some point.

What’s Involved When Fish Sleep?

Now that you’re aware that fish actually do sleep, you might wonder what their sleeping is like. For instance, what they do when they sleep, how long they sleep and even where they sleep.

Where do fish sleep?

All fish typically prefer to sleep in a specific location, either because of their safety, the lighting, or their comfortability. Depending on the certain specific of fish, some choose to hide when they sleep, while others tend to simply float anywhere in the tank. Some even hide in coral or algae, so if you can’t find them, then they’re probably sleeping.

What do fish do when they’re sleeping?

Every species of fish is capable of sleeping in different ways, even though they all typically experience the same resting mode. Most smaller fish experience their resting mode by either lying at the bottom of the tank or floating at the top of the tank. Not only that, but most fish also have restricted movements, and they don’t eat or swim when they’re resting.

In addition to that, most fish won’t react to movements that take place in the external environment around them. Other than lying at the bottom of the tank or floating at the top of the water, some other fish are capable of burrowing when they’re sleeping. More specifically, they likes to to bury themselves in between plants or in the hardscape that’s in the tank.

Other than that, some fish also like to simply drift in the tank. For instance, they refrain from swimming and limit their movement to only moving one of their fins every once in a while. Some even like to drift while they’re inside of a school, since they have the extra protection of the look-out of the fish that are surrounding them (3).

How long do fish sleep for?

It is difficult to tell how long fish are capable of sleeping for, since we do not have the capability of monitoring their brain activity. Some fish are able to remain inactive for long periods of time during the nighttime, and some are even known to rest all night long.

But this also seems to depend on the certain time of year as well as some other factors. Overall, this seems to vary among all species of fish, but most of them are known to rest whenever they have the time, which ranges from a few minutes per day to a few longer periods at nighttime.

When do they sleep?

Since most fish are diurnal, they typically choose to sleep and rest when they’re located in settings with dimmer lighting. Fish are capable of following circadian rhythms that are similar to ours, so they can follow a resting schedule of sleeping during the nighttime.

Not only that, but most fish have a pineal gland that releases melatonin when your body is located in darker surroundings. Given that, you shouldn’t have the bright light on over the fish tank 24/7, since your fish won’t be able to receive enough sleep.

How do you know when they’re sleeping?

Fish don’t close their eyes when they’re sleeping and in their resting mode, since they don’t have eyelids. Because of that, it can be difficult to know when exactly your fish are sleeping or not. But they do engage in certain behavioral pattens that can indicate that they’re taking a rest.

More specifically, it can be helpful to know that fish rarely sleep when they’re in fully-lit environments. They’re eyes are open, so when they are exposed to bright lights, they aren’t exactly able to sleep properly. Not only that, but you can tell whether your fish is sleeping or not mostly because of their body posture. More specifically, most fish choose a certain position to rest in, and it varies according to every fish species.

For instance, some fish lay down on the bottom of the tank or they float extremely close to the top of the tank with limited movement. Not only that, but some even lay on their side in between plants or on rocks when they’re sleeping. Other than that, you can also tell when your fish is sleeping if they dim the colors on their body. More specifically, some fish dim their colors while they’re sleeping in order to prevent predators and other dangerous fish from easily seeking them out and attacking them.

Furthermore, you can also tell if a fish is sleeping if they’re hiding in plants or other accessories in the fish tank. They will also have delayed response reactions, so if you walk close to them or try to scare them and they have either no reaction or a delayed response, then they were most likely sleeping.

Different Sleep for Different Fish

Just like other animals are responsible for staying awake and sleeping at different times, like raccoons, different species of fish are responsible for sleeping at various times throughout the day and night. Most fish are dinural though, so they typically choose to sleep during the nighttime. But other fish are nocturnal and choose to stay awake at night while hiding in darker environments when there’s light during the day.

Moreover, each species of fish presents different sleep patterns and sleeping habits. Some are known to shield themselves for protection at night, either by hiding in between plants or barriers, and other species of fish simply lay down anywhere that’s comfortable for them. Others drift, sleep with their school, or burrow themselves into a nest-like structure for additional protection and comfort.

Factors That Affect a Fish’s Ability to Sleep

Tank size

The size of your fish tank greatly affects the fish’s ability to sleep because tanks with varying sizes tend to have different lighting throughout the tank. More specifically, larger tanks with more water tend to disperse different quantities of light, since there is more water and the aquarium is deeper.

Decorations and accessories

If you have a tank that contains a large amount of decorations and hardscape dispersed throughout the tank, then the fish tend to sleep better. This is because they not only have a greater variety of places to sleep, but it’s also because the decorations tend to limit the amount of light that comes through the tank, creating shadows that the fish can hide in and rest.

Water temperature

The temperature of the water is also a large factor when determining how easy it is for certain fish to fall asleep. Certain species of fish require certain water temperatures in order to remain happy, healthy, and comfortable. Because of this, when the water is at a comfortable temperature for the fish, then they will have less trouble sleeping.

Availability of food

If you give your fish too much food, then the fish tank will be dirty, which lowers their ability to sleep in the lower quality water. On the other hand, if you don’t give your fish enough food, then they will remain constantly stressed about when they will eat next, giving them a more difficult time trying to fall asleep.

Other fish around them

Lastly, a fish’s ability to sleep will also be negatively affected if they’re placed in a tank where they are forced to compete with the other fish. Not only that, but a fish will tend to sleep less if they’re near other fish that they consider predators, since they have to constantly remain alert about their surroundings in order to prevent being attacked (4).

How You Can Make Your Fish’s Sleep Better

If you have fish of your own and are worried that they aren’t receiving enough sleep on their own, then there are certain things that you can do in order to make their sleeping habits a little easier. For instance, you should never tap on the glass or container of the fish tank. They can easily be woken up, and this limits the amount of sleep that they receive.

In addition to that, you’re recommended to refrain from placing a bright light over them 24/7. As mentioned previously, it’s difficult for fish to sleep in extremely lit environments, since their eyes are always open. Because of that, you should limit the amount of time per day that you have the fish tank light on. More than that, you may also find it beneficial to put your fish tank lights on a timer, since it’s healthier for fish to experience the same number of light and dark hours every day.

You may also find it helpful to install more plants and hardscape throughout the fish tank, since they will then have more space to rest in shadows, and they have more options of places to rest.

Frequently Asked Questions

Can fish develop sleep insomnia?

Since fish’s sleep patterns are difficult to study, we don’t have a specific indication of whether they’re deprived of sleep or not. But from observations, fish don’t seem to be negatively affected by sleep deprivation like we are. There are even weeks at a time that fish don’t even sleep at all, like migration and when they have eggs.

Do fish sleep upside down?

Fish generally aren’t comfortable when they’re sleeping upside down, even though some of them like to lay on their sides when they sleep. Because of that, if your fish is sleeping upside down, then either something is wrong or they’re not alive anymore. More specifically, most fish that swim upside down while still breathing typically have swim bladder disease, which is where they’re unable to control their movements or balance the weight of their body (5).

Can you wake fish up easily?

Yes. Practically al species of fish have excellent hearing, since they typically have two systems of hearing. More specifically, fish have bones in their inner ear as well as a lateral line located on the side of their body that gives them the ability to easily detect vibrations in the water.

In addition to that, vibrations and sounds are also amplified up to five times as much in the water, so tapping on the fish tank will definitely wake them up from their sleep (4).

Overall, fish are capable of sleeping, or at least resting in their own way. They don’t close their eyes and lay down like we do necessarily, but they are capable of limiting their movement and resting in such a way that helps them conserve their energy. Because of that, you can make your fish’s sleep better by giving them a great sleeping environment and refraining from purposely waking them up.