Fish are a particularly interesting case study in animal sleep, as their resting behavior has both similarities and differences to the sleep of land animals. Studying fish may reveal important answers about how sleep works, what function it serves, and how it has evolved over time.
A sleep-like state has been observed in many fish species, including sharks, rays, zebrafish, Bermuda reef fishes, Spanish hogfish, wrasse, parrotfish, goldfish, perch, and banded knifefish. To determine whether fish sleep, scientists consider both brain activity and behavior.
Sleep as Brain Activity
Scientists have identified specific hallmarks of brain activity that define sleep in many animals, including mammals, birds, and reptiles. For these animals, sleep includes stages of slow wave sleep and rapid eye movement (REM) sleep, with distinct physical correlates such as a slowing heart rate or reduced muscle tone.
In most animals, these sleep-related brain waves are typically recorded in a part of the brain called the neocortex. Fish do not have a neocortex, but researchers have identified sleep-like brain waves in another part of the brain called the dorsal pallium. These brain waves are similar to slow wave sleep and REM sleep in other species, and they occur along with characteristic changes in eye movement, heart rate, and muscle tone.
Sleep as Behavior
Under the behavioral definition of sleep, animals exhibit temporary periods of inactivity, specific body postures that vary depending on the species, and reduced responsiveness to their environment. Animals usually sleep more after a period when they have been deprived of sleep, which is known as a sleep rebound.
Zebrafish have regular periods of time where they stop moving, and they experience a sleep rebound after sleep deprivation. A number of fish species, including blueheads, Spanish hogfish, and wrasse, also exhibit periods of reduced responsiveness during which they can be easily handled and moved by scientists. Similarly, sleeping goldfish and perch do not wake up when touched on their tail, and sleeping nurse sharks only respond to stronger-than-usual stimuli.
Why Do Fish Sleep?
Scientists do not yet know why we evolved to sleep. There are two main theories regarding the origins of sleep.
Restorative Sleep Hypotheses
Advocates of the restorative sleep theory propose that sleep enables the body to physically repair itself, bolster the immune system, and consolidate new memories. In support of this theory, researchers have found evidence that sleeping fish have a lowered metabolism as well as changes to their heart rate.
Adaptive Sleep Hypotheses
The adaptive sleep theory proposes that sleep evolved as a function of an animal’s environment, in order to keep animals out of danger and allow them conserve their energy when it makes sense.
How Do Fish Sleep?
The sleep habits of different species can vary according to their exposure to light, their place in the social hierarchy, their physical traits, and their environment.
Nocturnal vs. Diurnal Fish
Many smaller fish species such as rainbow wrasse, three-spot wrasse, and zebrafish are diurnal, meaning they sleep at night. When asleep, zebrafish stop swimming and hover motionless unless a serious disruption forces them to wake up. Their coloring also changes, which may help disguise them from predators. By contrast, sharks are often nocturnal or crepuscular, which means they sleep during the day and venture out to hunt at night or twilight.
Cave-Dwelling vs. Surface-Dwelling Fish
Fish species who live in dark environments may need less sleep than those who live closer to the surface. For example, Mexican cavefish that evolved to dwell in caves have smaller eyes, or no eyes at all, and sleep much less than sighted Mexican cavefish who live in rivers closer to the surface.
How Do Fish Breathe During Sleep?
Many species of bony fishes, sharks, and rays breathe by opening and closing their mouths to push water over their gills. This process enables them to float still for a long time, breathing while they sleep.
Other species of fish must keep swimming in order for the water to flow across their gills. Depending on the species, researchers believe they may either swim slowly within a confined perimeter, sleep in currents, keep their fins moving, or sleep with only part of their brain to keep breathing while they sleep.
Sleep in Fish vs. Land Mammals
Mammals show a clear distinction between REM and slow wave sleep. Although research so far is limited, studies on zebrafish suggest they experience two markedly different sleep stages that alternate back and forth. The first is characterized by regular pulses of nervous system activity, similar to slow wave sleep in mammals. The other is more similar to REM sleep.
Researchers have yet to determine if fish experience true REM sleep. During REM sleep in mammals, heart rate and breathing become irregular, closed eyes move rapidly, and brain activity increases its pace. Experiments on zebrafish have found they display many of the same characteristics, but without the rapid eye movements.
Zebrafish also appear to share some of the same hormones and possibly some of the same pathways that regulate sleep in humans.
Where Do Fish Sleep?
Some fish sleep while swimming, some float motionless, and others hide out of sight from predators. For example:
- Rainbow Wrasse: As dusk falls, these fish burrow into the sand and stay there motionless until dawn.
- Parrot Fish: These fish float under coral for protection and create their own bed out of a mucus cocoon.
- Sharks: Sharks may retreat to caves or the ocean floor. Some shark species prefer to sleep alone, while whitetip reef sharks like to sleep close together.