How Fish Sleep
Scientists do not know for sure if all animals sleep, but it is believed that the vast majority of them do, including fish. The fact that fish always have their eyes open, and that their anatomy is so different, has led to the belief that these creatures cannot sleep. But in reality this is not the case.
Although it is not the same type of sleep that we humans experience, fish do sleep. It has been observed that most species exhibit a period in which activity and metabolism slow down to restore the body and conserve energy.
Read on below to learn everything you need to know about the subject: how, when, where and why fish sleep, what signs indicate that a fish is asleep, how different species sleep and the importance of sleep for these animals, among other frequently asked questions.
How Do Fish Sleep in an Aquarium or Fish Tank?
The first thing to keep in mind is that the way a fish sleeps depends on the particular species. For example, some fish will head for the bottom of the aquarium, tank or pond, while others sleep floating almost inert near the surface.
There are also species that sleep completely immobile and others that move a fin occasionally to maintain their balance.
But in general, there is one thing they all have in common, and that is that fish sleep is described as a dreamy or restful state in which they become very still and their breathing slows down. Unlike other animals, they cannot close their eyes because they lack eyelids.
Despite being asleep, fish remain alert to danger. This allows them to react if they sense the presence of a predator. Even so, some have been known to “sleep so deep” that they do not move when pulled from the water by hand.
These creatures do not seem to follow such a strict circadian rhythm. They apparently adjust their sleep patterns according to lighting, water temperature and food availability.
Did you know that…? Zebrafish have been found to be the only fish that sleep similarly to mammals. After monitoring the brain and body activity of these fish, researchers identified attributes such as slow-wave sleep and deep sleep.
Where and When Do They Do It?
The location for sleeping differs from one species to another. Some fish lie on the bottom or bury themselves in the sand; some hide in small caves and under rocks; others prefer to float, move motionless near the surface or settle near a plant, coral or piece of wood.
A very curious case is that of the parrotfish, which sleeps surrounded by a cocoon or membrane of transparent mucus that it secretes. It is believed that this is a defense mechanism that protects it against predators and parasites.
Seeing a motionless fish floating or stuck to the bottom of your tank need not be cause for alarm. It is most likely just dozing.
But When do Fish Sleep?
Most aquarium fish are said to be diurnal, meaning they are most active during the day and rest at night. However, some species are nocturnal, hence they spend the night prowling around and fall asleep during the day.
This can be a problem in certain cases, for example if small tetras, such as neons, and catfish are kept in the same aquarium or pond. While the neons are sleeping on the bottom at night, the catfish will be swimming around looking for an easy meal.
Summary Note
Fish have the ability to sleep and rest. This occurs during periods of notable inactivity, in which they often hide, float or remain very still near the bottom. Given the absence of eyelids, they cannot close their eyes, nor do they exhibit the rapid eye movement of REM sleep.
Fish sleep is still an area of active research. Although they do not sleep in the same way as terrestrial mammals, they are known to experience periods of rest with our same restorative functions.
Diurnal and Nocturnal Fish: How Do They Know It’s Time to Sleep?
A key element that determines how fish know what time to sleep is light.
Diurnal fish, which are active during the day and at rest at night, cannot see well in the dark. When the sun goes down, they instinctively hide and adopt a resting posture.
On the other hand, nocturnal fish are more active at night. During the day they prefer to stay hidden in a shady place, waiting for darkness to come and feed.
But regardless of whether a fish is diurnal or nocturnal, the most important thing to keep in mind is that these creatures need a light-dark cycle. Without it, their bodies don’t know when to activate or when to rest.
If your fish live in an indoor aquarium, it is essential that you control the intensity of light they receive and the time it remains on. You should provide them with a day-night cycle, so that you mimic what happens in nature.
Expert Advice
- Never leave the lights in an aquarium or fish tank on 24 hours a day. This can stress the fish, as the light affects their normal sleep/wake cycles.
- It is always healthier for fish to receive a consistent number of hours of light and dark. If you are not able to turn off the lights at a set time, consider using a timer.
How to Tell If a Fish Is Asleep? the 7 Basic Signs
Again, this depends on the species.
To begin with, most fish do not sleep as long as they are receiving any light source. For example, some fish in your tank may not rest until the house lights are completely off. Not being able to see them makes it more difficult to tell if they are sleeping.
But generally speaking, there are several signs that help identify when a fish is asleep and when it is awake.
1. Inactivity for a Period of Time
The fish stops swimming and remains almost inactive. It may remain completely still, or remain floating in a quiet corner, moving only the tail or a fin occasionally. Mouth movements and gill movements become almost imperceptible.
Interesting fact. Some large fish, such as tuna and sharks, have to swim all the time in order to breathe. That suggests that they sleep with only part of their brain, just like dolphins.
2. Body Posture
Depending on the species, fish may float with their head down or up, lay along a leaf/branch, and even lie on their side on the substrate. In addition, they are likely to have a drooping tail.
3. Position Near the Bottom
Many fish lay right on or near the bottom when they are sleeping. This pattern is very common in betta fish, tetras, goldfish, tiger barbels, guppies, danios, etc.
4. Search for a Hiding Place
This is a very common behavior associated with resting. They can be seen hiding among plants, in small caves and crevices, or behind decoration.
5. Lack of Response
Sleepy fish do not respond to what is going on around them as they normally would. For example, they do not swim hurriedly to the front of the aquarium or tank if you approach to feed them.
6. Faded Colors
Another change that indicates that a fish may be asleep is a decrease in the intensity of its colors. Many species are known to attenuate color during rest.
7. Cyclical Behavior
Finally, you may notice that the fish exhibits the above behaviors on the same schedule each day, probably when it is no longer receiving light.
What Can Affect the Sleep of Fish?
All those who keep fish at home, and especially first-time aquarists, should know that there are some factors that alter their sleep pattern.
- Stress. Stress caused by inadequate temperature, poor quality or insufficient water can cause fish to stop sleeping.
- Feeding. They may also change their sleep/wake patterns depending on the availability of food. Some aquarium fish may become nocturnal if food is available at night, or diurnal if it is only available during the day.
- Diseases. Sick fish often change their sleep patterns and duration. It is known that they may sleep less in an attempt to appear healthy and therefore less vulnerable.
- Reproduction. These creatures can go for days without sleep when they are protecting their eggs.
Quick Answers to Frequently Asked Questions
Why Do Fish Sleep?
Much remains to be understood about sleep in fish, researchers report. While it is not known for certain why they sleep, it is believed that these animals enter a low-energy mode to rest, recover, heal injuries, fight disease and grow.
Unlike humans and other mammals, fish do not need to convert short-term memory to long-term memory. They do not experience REM sleep and apparently do not dream either.
Do All Fish Sleep?
The vast majority of fish do sleep, but there are some exceptions. Scientists believe that some species do not sleep, including fish that live in caves, in the deep ocean, and those that are blind and swim continuously.
This seems to be due to the lack of visual information. By not receiving the visual stimuli of the day, their adaptive function for brain neuroplasticity is minimal and, therefore, they do not require the rest provided by sleep.
How Many Hours Do Fish Sleep?
It is very difficult to know the exact number of hours a fish sleeps. At the very least, we would have to observe it directly to try to estimate how much sleep it has had in a 24-hour period. However, it is believed that fish need between 9 and 10 hours of sleep. Some may sleep up to 12 hours if there is sufficient darkness.
Generally speaking, the length of time fish remain at rest depends on the species, how active they have been and other environmental factors.
For example, the sleep cycle of fish living in a home aquarium or fish tank is determined by the interior lights. If the aquarium is dark most of the time, then the fish are likely to sleep longer hours, and vice versa.
How Do Guppy Fish Sleep?
The guppy or lebistes is a freshwater fish that prefers to sleep in the dark. It usually sleeps during the night, when there is little or no light source. A sleeping guppy remains floating just above the substrate or on plants and decoration, although it can also be seen inactive very close to the surface.
When in absolute resting mode, guppies may appear dead and discolored. Their respiratory rate decreases and they may become less sensitive to sound. Once the lights are turned on, they become active and regain their colors.
Note: If a guppy fish floats to the surface during the day it is most likely not asleep. Instead, this could indicate that something is wrong.
How Do Goldfish Sleep?
Goldfish are diurnal; that means they are active during the day and sleep at night. During sleeping hours, the goldfish becomes motionless, inactive, always in one place near the bottom. It is possible to see some movement, but it will be almost imperceptible.
These fish make small flapping movements to keep themselves stable while they sleep.
Likewise, they tend to have their heads pointing slightly downward and their colors fade.
How Do Betta Fish Sleep?
Betta fish, also known as fighters, are diurnal. They remain active during the day in the aquarium and sleep at night. This usually occurs once the lights have been turned off.
Bettas usually sleep in different ways, for example inactive on the gravel, hidden inside the decoration or between two objects, floating on the surface, resting on a leaf or on their side.
Some even adopt very funny postures, even wedging their bodies under a rock to turn themselves upside down, or settling into an L-shape to take a short nap.
How Do Angelfish Sleep?
Angelfish sleep during the night, when they become less active. Once darkness comes to the aquarium, this fish descends to the bottom and remains motionless there. Its body colors fade, resulting in an obvious fading of its spots and stripes.
Some angelfish may move while sleeping, but these movements are usually slight and lethargic.
Do Fish Sleep With Their Eyes Open?
Yes, strange as it may seem, fish can sleep with their eyes open at any time, night or day. These creatures cannot close their eyes because they lack eyelids.
For fish, sleep is more like a period of rest and recuperation, similar to the “daydreaming” we humans experience.
What Are the Fish That Do Not Sleep?
According to researchers, the only fish that do not sleep are those that are blind and live in dark caves or in the depths of the ocean. Being constantly in the dark, these species do not know when it is night and when it is day.
The rest of the fish, which always sleep in one way or another, may temporarily stop sleeping in very specific situations. For example, when they need to care for their young.
In a general sense, all pet fish living in aquariums, fish tanks and ponds sleep.
Do Fish Suffer From Insomnia?
There is still not enough information on insomnia in fish. The only exception is the zebrafish, a species whose sleep patterns have been extensively studied. In the specific case of this fish, it has been observed that the absence of darkness prevents it from falling asleep.
Apparently, light inhibits the sleep-promoting hormone melatonin in zebrafish, just as it does in humans.
Do Fish Sleep During the Day or at Night?
Simply put, it depends on the species. Most fish sleep during the night because they are diurnal, but there are species that sleep during the day because they are more active at night.
For example, angelfish, goldfish, guppy, betta, blue surgeonfish and parrotfish sleep at night or when the environment is dark. However, cardinalfish, plecos and catfish are considered nocturnal, as they come out to feed at night and rest during the day.
Then, there are the crepuscular fish species. These are most active during dawn and dusk, hence they can sometimes sleep both day and night.
Is It Normal for a Fish to Sleep on Its Side or the Other Way Around?
No. When a fish sleeps or remains inactive with its body sideways or upside down, i.e. belly up, it usually indicates that something is wrong. Specifically, goldfish are well known for this behavior, but that does not mean it is normal.
The most common cause is swim bladder disease, which causes the fish to be unable to control its movements and balance its weight while sleeping.
Do Fish Dream?
Some studies have suggested that in addition to sleeping, fish may also dream. But the truth is that this theory has not been confirmed. Other researchers claim that fish do not dream, because unlike us, they do not experience REM sleep and do not have such a complex brain.
Do Fish Sleep With the Light On?
The way fish sleep differs from species to species, but in a general sense it is important to provide them with hours of light and hours of darkness.
It is not recommended to have the aquarium, fish tank or room lights on 24 hours a day. Too much light can cause stress, especially for species that need a dark environment to sleep.
How Do I Make Sure My Fish Get Enough Sleep?
A good recommendation to ensure that pet fish get enough sleep is to set a timer that regulates the aquarium lighting. This will help create a more effective light and dark cycle, as well as a healthier sleep routine.
Sleep is very important for fish. When a fish does not get enough sleep, it can become very stressed. In turn, constant stress makes it more vulnerable to disease and shortens its life expectancy.
How Do Other Fish and Marine Animals Sleep?
Sharks
Sharks are a type of fish that needs to swim to breathe. That is why they cannot stay still while sleeping like other fish. It has been observed that sharks keep their fins and tail in motion, while turning off a part of their brain. This allows them to recover in phases.
Dolphins
Dolphins can sleep floating above or below the surface, but only for shorter periods. Researchers have discovered that, unlike humans, dolphins’ brains remain partially alert.
Whales
Whales also often sleep while swimming. These creatures turn off half of their brain, while the other half remains active to ensure breathing and movement. During the sleep phase, the youngest whales are guided by the mother.
Final Thought
Did you find the content of the article interesting? Although they do not do it in the same way as other animals, fish do sleep. This can be described as periods of rest in which they save energy, recover their bodies and grow.
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