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Research has shown that most animals do sleep in some form. While REM sleep and dreaming might only be reserved for mammals and birds, all other animals also require some rest. Even jellyfish, one of the oldest known multicellular organism, with the most simple nervous system, has been observed to experience some form of sleep. They have been around for more than 500 million years, long before the dinosaurs even existed. Because of this discovery, it is proposed that sleep is widespread among all of the animal kingdom, and it serves a crucial purpose of restoring body and mind, as well as forming memory and learning.
When we think about how sleep works, we often imagine laying down in a comfortable position in our bed, with our eyes closed, disconnected from the world. Because of that image, people often have a hard time imagining if animals can sleep. It’s easy for mammals, as we’ve seen tons of cute sleeping dog and cat videos, but when it comes to animals that we are less familiar with, we don’t have a clue. If your kid has asked you if their pet goldfish sleeps, and you didn’t know what to say, don’t worry, we have all of the answers for you.
How Do Fish Sleep?
Fishes do not have eyelids, so they can’t close them. One of the purposes of eyelids beside protecting an eye is to spread the fluid across it, so it can function adequately without drying out. When you are always in the water, you don’t have that problem. Fishes also have a far simpler central nervous system than mammals, so they don’t have specific regions of the brain that we do. Take neocortex, for example, that plays a considerable role in sleep. It is a part that produces certain brain waves that are distinctive in different stages of sleep in humans. Also, it is very active while we are dreaming in REM sleep, which means that fish don’t experience REM sleep and that they most likely don’t dream. We say most likely because we can’t be a hundred percent certain, considering how complicated even the fishes’ central nervous system is. Future scientific research might show that they, in fact, experience dreams, but that the other parts of the brain are involved.
Because fish are lacking these structures that would undoubtedly say that they are sleeping, we had to define other parameters to describe the way they rest. These are:
- A state of prolonged inactivity
- A typical resting posture
- The repetitive pattern of this inactive behavior during the 24-hour cycles
- Reduced perception of external stimuli
According to these criteria, sleep in fish is a period of inactivity, almost always with the same posture and in the same location, at the same time every day, while having decreased sensitivity to outside disturbances. Another criterion might be the presence of “rebound sleep.” The induced absence of sleep-like behavior for some time should mean that the inactivity will be more present during the following days that fish is left alone.
The Difference in Sleep Habits Between Fishes
There are over 30,000 species of fish, so naturally, there is a lot of variation in the way that they sleep. Sometimes, even those four parameters that we have described are not enough to tell if the fish is sleeping or not, cause they might not fulfill all four, but it is apparent that they are resting.
Some quietly float on the surface or near the bottom, while others drift round with an occasional flick of a fin to keep them going. Some get the needed rest under a rock, in a whole, or in hiding between marine algae. There are some that even create nest-like structures in the sand and the ones that bury themselves when going to sleep. Some parrotfish have glands inside the gills that secrete mucus. This way, when going to sleep, they produce a cocoon, that protects them from parasites, but it also blocks their smell from potential predators lurking nearby.
Some species like green bromis sleep in branching corals, and they have evolved an interesting symbiotic relationship. While fish is sleeping, it energetically moves its fins, increasing the flow of oxygenated water around the corals. Because algae that live there can’t photosynthesize and produce oxygen during the night, it looks this peculiar sleep behavior is needed for the coral survival.
Spotted wolfish is a beautiful example of how fish behavior can be similar to our sleep. Because it doesn’t have pelvic fins to stabilize its body, this fish flips on its side while sleeping. Others, like bluehead wrasse, are so unresponsive during the resting period, that scientist stated that they were even able to pick them up by hand and bring them to surface without waking them up.
Some fish even catch some rest while schooling. There are a lot of fish in the school that can spot the predator if it comes nearby, so some members decide to turn on autopilot and snooze for a bit. Another fascinating thing about fish is that they can produce electrical waves and communicate by them, so electrical signal can fastly spread between school and alarm everybody about the danger.
As the quality and quantity of light can vary drastically with the depth of the water, other factors can tell fish when is the time to rest. The temperature of water, availability of food, and the presence of predators are all valid signals for a fish for adjusting their sleep/wake rhythm. Some even can switch between sleeping at night or day, depending on these other factors.
The most studied fish is zebrafish (Danio rerio). It’s a great model organism as it shows clear signs of sleep, it stops moving, it’s breathing and heart rate slow down, and it takes much longer to respond to outside stimuli. It’s also observed to get a rebound effect, meaning that when deprived of sleep for some time by the researches, it tries to catch up on the lost sleep when it is left alone in the dark again. However, if the light stayed on, the rebound effect has not been observed. The study of zebrafish is providing some good info about how sleep works, and how the sleep deprivation can affect animals as well.
Do All Fish Sleep?
Most fish are diurnal animals, meaning that they are active during the day and sleep at night. However, there are some that are nocturnal, so they are active during the night and resting during the day. Weirdly, there are some fish that seem not to sleep at all.
Bluefish and mackerel continuously swim in the big seawater aquaria, and even though they seem to slow down at night, they remain responsive to outside disturbances and the introduction of food. However, scientists argue that this state is induced due to the high stress of living in captivity.
There are some fish that have a prolonged period of sleep absence, or they seem to be completely lacking the rest:
- Tuna and some sharks seem not to sleep at all. Scientists are still not sure why that is, but they hypothesize that it is because of the role that sleep has on memory and storing information. Rest serves a purpose to embed memories in our brain and to process all the stimuli that we get during the day. Since those fishes sleep in a rather boring environment, in the deep ocean where the scenery doesn’t change much, it is suggested that they don’t need to relax. Some cave-dwelling fish are also restless, and it may be for the same reason, lack of visual sensory input. However, some scientists are not sure that these creatures are entirely sleepless, and they attribute it to a lack of observation. They propose that some sharks and tunas might have bursts of microsleep, that are observed in some other animals.
- Some fish lack sleep during the juvenile stage. Mozambique tilapia is one of those, and it is observed that it doesn’t sleep for the first 22 weeks of its life. Since tracking rest patterns for a long time in fish is not the most attractive topic of research, scientists still need to figure out whether this is a common phenomenon. Larvae and adults of zebrafish seem to follow the same sleep patterns.
- No rest during migration. Fish seem to be occupied during the period of movement, so there is no time to catch some snooze time. It’s interesting that scientists have created an experiment where they simulated the autumn condition during the winter to see how the tautog would behave. They usually migrate during the fall, so they started to be more active and didn’t sleep. The juveniles weren’t affected which is consistent since they don’t migrate; only adults do.
- Lack of sleep during spawning or parenting period. Some species like the mentioned tautog, bullhead, minnows, yellow perch, and many others seem to be active continuously through spawning season. Others do that when it’s time to care for their eggs. Because eggs need the flow of oxygen all the time, fish look after them and fan them to provide a continuous stream of oxygenated water.
Why Do Fish Sleep?
It seems that this resting period in fish has the same crucial restorative function as it does in humans. Sleep is a time when our brain can do a little maintenance, and it can send signals to repair muscles and other tissues in our body. It’s also a period necessary for our memory storage and learning, and it seems that it plays the same role in fish as well. It’s just that their brains are much simpler than ours, so the effects are not as apparent. Because of the simplicity of their central nervous system, they also don’t go through the same sleep stages as we do, and they don’t experience REM sleep and don’t seem to dream.
Fish can also switch the time of the day when they sleep depending on the outside conditions. Water temperature, the presence of predators, and availability of food can trigger this. Atlantic salmon is an example of this, as they turn nocturnal in colder waters. Fish are not endothermic organisms, like mammals and birds, and their body temperature depends on the temperature of water. Their muscles also produce heat while swimming, so they have to keep move to be warm. Some fish like tuna produce so much heat that they are considered partially endothermic. When the salmon gets in the colder waters, they slow it down (as they turn a bit sluggish) so being active during the night provides them some safety against predators.
Another valid question about our pets, since they are not in the wild is how they sleep in a tank. And do our pets like goldfish, guppies, and betta fish sleep at all?
Guppies like sleeping in the dark, so they mostly rest at night when all the lights are off. They usually hover in one place just above the bottom. Sometimes they can float on the surface, but if this happens during the day, it is an indication of some problem.
Bettas sleep in a variety of ways, some float in one place, while others can rest on a substrate or plants in their aquarium. Their gill movement is considerably lower during this period.
Goldfish are diurnal, meaning that they are active during the day. They’ll most likely sleep at night when you turn all your lights out. These pets are low maintenance, and despite the popular belief, they are quite smart. Forget about that myth that they only have a few second memory span; they can remember things for months, even a year. They can also be trained to respond to different kind of music, color, or other sensory cues. In one interesting experiment, scientists thought goldfish to press a lever to get food. Then they limited the time during the day when they would get food by pressing a bar to one hour. It took them very little to learn this, and not only did they use this opportunity to get extra food, but they also didn’t touch the leaver during the hours that it didn’t have a rewarding effect, meaning that they very much have a developed memory and a sense of time.
If your fish is sleeping upside down or on its side, know that this isn’t a usual behavior, unless they told you that it is how that species rests. Other than that, it most likely indicates a health problem, and you should take your pet to the vet.
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Dusan is a biologist, a science enthusiast and a huge nature lover. He loves to keep up to date with all the new research and write accurate science-based articles. When he’s not writing or reading, you can find him in the kitchen, trying out new delicious recipes; out in the wild, enjoying the nature or sleeping in his bed.