Danielle writes in-depth articles about sleep solutions and holds a psychology degree from the University of British Columbia.
Sleep plays an essential role in our overall functioning, development, and how long we live. Without enough sleep, our health and cognitive performance suffers. The same goes for animals, including fish.
Many types of fish appear to sleep, but fish sleep differs from what we usually think of as sleep. Researchers have not been able to measure the familiar brain wave patterns that characterize human sleep and the sleep of many other animals in most fish, so researchers often refer to fish sleep as rest. Also, since most fish do not have eyelids, they cannot close their eyes during this rest.
Fish appear to be more alert than humans are during sleep, which may give them more time to react to potential threats in their environment. Still, fish do slow down metabolic processes in their resting state, much like humans. They physically slow down as well, with some fish floating in place.
Since fish do not look very different when they are asleep, it can be difficult to determine whether a fish is resting or awake. However, during a state of rest, fish are much less responsive. Some may appear to stop moving completely, and can even be touched or handled without waking up.
Moreover, some fish are more vulnerable to being attacked by predators at night, which suggests that they are diurnal, or resting and less alert at night. Studies have found that certain fish species experience more deaths in the first two hours after sunset, which indicates that these fish tend to fall asleep at that time.
A lack of responsiveness is often a sign of fish sleep, but researchers have found additional markers of sleep that fish share with humans and other animals. For example, studies show that melatonin may regulate the sleep-wake cycles in zebrafish. Zebrafish also appear to have distinct stages of sleep like humans, and follow a circadian rhythm.
Humans cycle through four stages of sleep, including two stages of light sleep, one stage of slow-wave, deep sleep, and one stage of rapid eye movement (REM) sleep. Although fish do not have a neocortex like humans, researchers were able to monitor the brain activity, heart rate, and eye and muscle movement in zebrafish to document two stages of sleep that are similar to slow-wave sleep and REM sleep.
When fish do not get enough sleep, they seem to be vulnerable to the effects of sleep deprivation. After a night of sleep deprivation, zebrafish experience a sleep rebound, in which they catch up on sleep.
This resting state is so essential to fish that they will find ways to maintain a bare minimum of sleep, just like other animals. For example, if fish are subjected to experiments in which their sleep is interrupted, they will eventually nod off into microsleeps, managing to obtain at least 5% to 10% of their normal amount of sleep.
Many fish follow a regular sleep-wake cycle, similar to humans and other animals. The circadian rhythms of fish appear to be sensitive to light, similar to the human circadian rhythm that prompts us to feel alert during daylight and sleepy after dark. In one study of zebrafish, researchers interrupted the fish’s sleep, either via light exposure or an electrical or mechanical stimulus. Both types of interruptions woke up the fish, but only the light exposure significantly reduced the amount they slept for days at a time.
Light-dark cycles are a common regulator of circadian rhythms in many animals. But there are fish that cannot see light, such as deep sea cavefish. Cavefish live so deep in the sea that they live in darkness, and many have evolved without sight as a result. Even so, they display behaviors that are consistent with having a circadian rhythm, leading researchers to believe other environmental factors, like food, may help regulate their sleep-wake cycles.
The sleep habits across and within different fish species are as varied as those of other animal species. Researchers hypothesize that fish’s sleep needs may have evolved according to factors in each population’s environment. For example, surface cavefish sleep significantly more than those of the same species that live in deep sea caves.
Fish species can also be diurnal, which means they generally sleep at night, or nocturnal, which means they are generally active at night. For example, nocturnal sharks rest during the day and then swim and look for food at night.
Fish need oxygen to survive, and they get it as water passes against their gills, which requires a certain level of movement. Some fish can receive oxygen while in a nearly stationary position by facing a current, staying afloat and moving a fin every now and then as water passes around them. Other fish, like some sharks and rays, need to make larger movements to move a sufficient amount of water over their gills. Due to unihemispheric sleep, these larger fish can keep swimming while they sleep.
Unihemispheric sleep allows a fish to put one half of their brain to sleep at a time. The other half stays active, allowing the fish to keep swimming, albeit more slowly than when they are awake. If they had eyelids, the eye connected to the sleeping side of the brain would close while the other stayed open, as it does with many birds and whales.
Some fish occasionally enter estivation, a state of suspended animation that is similar to hibernation, but occurs in dry rather than cold climates. Fish, reptiles, and amphibians may estivate during periods of food or water deprivation. During estivation, their metabolism slows down drastically, enabling them to survive for a dry season, or even years.
Cavefish do not enter estivation, but they do change their sleep needs based on their access to food. When they have less access to food, they increase their sleep. When there is a lot of food for them to forage, they may sleep less.
Where a fish sleeps depends on the fish species. Coral reef fish may float in place and simply stop moving when they want to sleep. Nurse sharks may rest along the sea bottom or near a rocky cave.
Some fish do more to protect themselves while they rest. Certain species of coral fish retreat within the reef before falling asleep. Parrot fish may also sleep under coral, or they may create a protective mucus cocoon. Other fish, like rainbow wrasse, may burrow into sand or mud for protection. Some sharks rest in a group.
There are several signs that may indicate that your fish is sleeping:
The next time your fish is less active, you may want to look out for these signs to determine if they are asleep.