Do Fish Sleep?
Everyone needs sleep. Each night, most people perform a sleep ritual: they change into pajamas, crawl into their soft, comfy beds, close their eyes, and enter into a restful state. Their hearts slow down, they begin to breathe slowly, and their muscles become relaxed. We spend about 8 hours a day sleeping, which amounts to one third of our lives.
Sleep means different things to different forms of animal life. The Saunders Comprehensive Veterinary Dictionary sums it up perfectly: “A period of rest during which volition and consciousness are in partial or complete abeyance and the bodily functions partially suspended; a behavioral state marked by characteristic immobile posture and diminished but readily reversible sensitivity to external stimuli.”
Most animals have some daily pattern of rest and activity, and in many species these daily cycles are similar to humans, since they attain nourishment and perform tasks during the day and rest at night.
This leads to the question, “Do Fish Sleep?”
As you look into your aquarium or a pond from the surface of the water, it can be difficult to tell what the fish inside are actually doing. It is believed that fish are no different from us, although fish sleep is a controversial and intriguing subject. In fact, fish sleep behavior has been the topic of several books and scientific studies.
How sleeping fish appear varies with the species. Some fish keep very still, experiencing a quiet period (quiescence) that you might call sleep. Scuba divers often handle reef fish in the middle of the night without startling them and can even lift some species out of the water before they awaken. Tropical freshwater fish in home aquaria appear to be resting immediately after turning the lights on in a room that has been darkened for several hours. Unfortunately, fish have no eyelids, so it is difficult to tell whether they are asleep or not.
Why We Sleep
No one knows for sure why creatures, including fish, sleep. However, there are two distinct theories:
Sleep Has a Restorative Function
It is possible that sleep helps the body recover from all the work it did while it was awake. In most species, sleep provides the body with time to restore and repair itself. There are extensive and ongoing studies in humans that show the critical importance of sleep on restorative function. In fact, some functions occur better or only during sleep, which is why sleep is so important for good health. For example, it is during sleep that tissue repairs, muscles grow, and protein synthesis occurs.
Those who get an insufficient amount of sleep are at an increased risk of diseases like hypertension, diabetes, obesity, depression, infection, cancer, stroke, and Alzheimer’s. Just as quantity of sleep is important, so is quality. Better quality sleep is associated with better health. As part of the restorative functions, sleep optimizes neuroplasticity. This is a term used to refer to the ability of the brain to change and rewire itself. This allows us to process what was seen and done and the ability to continue to learn in response to information or experiences. It is believed that these functions also occur in fish to some degree. Scientists are actively working to learn more about the specific restorative functions that occur in fish during sleep.
Sleep Has an Adaptive Function
A common evolutionary theory of sleep is that it is an adaptive function that occurs in order to protect oneself or conserve energy. When fish move into their hiding places to rest, this lowers their risk of getting eaten by predators. During sleep, they also lessen their physical demands, which allows them to conserve resources.
Understanding Fish Sleep: Different Strokes
Being asleep can mean different things to different fish. It’s easy to see when people, mammals, and even birds sleep, because they close their eyes and remain motionless. Some fish and amphibians reduce their awareness, but do not become unconscious like the higher vertebrates. When you add to this the fact that they don’t have eyelids, it makes it even more difficult to see if fish sleep.
Fish have time periods when they become less aware of their surroundings, but their brain waves do not change, and they do not exhibit REM sleep. They aren’t quite asleep, but they don’t seem to be fully awake either.
Some fish undergo a yearly sleep cycle. They hibernate and their metabolic rate slows down. Although fish do not hibernate like mammals, as environmental temperatures fall, their metabolic rate and activity decrease, and they go into a stupor and stop feeding. Sleeping fish usually move toward the bottom of the pond.
Another common practice in fish is estivation, which is a state of torpor or dormancy during hot, dry periods to protect from dehydration. The African lungfish protects itself from the dry season by cocooning itself in the mud of the riverbed. Walking perch bury themselves in mud, leaving only an airhole open, and breathe by means of their lungs. Carp spend the winter partly buried in lake mud, and in tropical countries, many fish sleep, or estivate, through the summer months when swamps and rivers dry up.
One of the gobies of the Ganges River delta digs a burrow and sleeps through the dry months with only the tip of its tail touching the water. It apparently breathes through its tail.
Some fish make elaborate preparations for sleep. In David Feldman’s book When Do Fish Sleep?, a scientist describes the nightly ritual of a tired parrotfish that lives in reefs near shore. The parrotfish squeezes into a crevice on the reef. Once settled in, it begins oozing a jelly-like mucus, which forms a protective membrane over its body, and then it nods off into a deep sleep.
Other fish remain motionless in the water during the night, while others, like rockfish and grouper, don’t appear to sleep at all. They rest against rocks, bracing themselves with their fins. Some freshwater fish, like catfish, swim up under a log or riverbank for shelter during the day.
Finally, some fish don’t hide the fact that they take an occasional nap. One of the favorite habits of the clown loach, which can be quite alarming, is resting on the bottom of the aquarium on their side. They appear as though they are dead or sick, but this is just one of the positions that they adopt when resting.
Many fish will lie motionless at the surface of the water or near the bottom when sleeping. Their breathing rates will slow, and they often appear groggy or slow to respond. For example, tilapia, zebrafish, and the swell shark can become unresponsive at night. Some fish will spend their time in the coral of a coral reef, and their bodies will appear to be moving and swimming while they are sleeping.
NOTE: Fish should not swim upside down or sideways. If you see a fish doing something like this, it is likely that they are having a medical problem. For more information, go to our fish medical library.
What Else Can Impact Fish Sleep Patterns?
There are several things that can impact fish sleep patterns:
- Stress caused by poor water quality or low water levels can cause fish to skip sleep.
- Fish can also change their sleep/wake patterns based on availability of food. For example, some aquarium fish will become nocturnal when food is consistently available at night or diurnal if food is readily available during the day.
- Fish species guarding their eggs or migrating will go without sleep for days.
- Water temperature and availability can impact sleep patterns. Some species of fish, such as salmon, are more active during higher daytime temperatures.
- Illness in fish can cause changes to sleep patterns and length of sleep. For those that sleep less, it may be a protective mechanism where they want to appear “healthy” and thus less vulnerable to predators.
Do ALL Fish Sleep?
It is probable that fish do sleep in some form, whether it’s slowing down or coming to a complete stop. However, when fish sleep, even the slightest ripple in the water will disturb them.
There are a few species of fish that are believed not to sleep. This includes cave-dwelling fish that are blind and swim continuously, as well as fish that live deep in the ocean. Many scientists believe that, due to lack of visual information to process, the adaptive function for brain neuroplasticity is minimal and, therefore, they don’t require the rest provided by sleep.