Nighttime Rituals: Do Fish Really Sleep?

It’s easy to tell if a person or animal is sleeping. They lie motionless, and their eyes are closed. Fish, however, always seem to be on the move. They don’t have eyelids, so a fish’s eyes never close. It’s understandable why many people may believe that because of these reasons, fish don’t ever sleep. It may surprise you to learn that fish do sleep. However, a fish’s sleep looks much different from that of a human’s or other land animals.

When you go to sleep, you close your eyes and lie down. Many people follow a bedtime routine diligently, while others might flop onto the couch after a long day. Most healthcare professionals recommend 8 to 10 hours of sleep a day. Fish sleep is much different from the sleep that we are familiar with. Humans and land animals need eyelids to protect their eyes from debris and dehydration. Fish live in water and therefore have no use for eyelids. This means that when a fish sleeps, its eyes stay open. Fish also sleep in shorter intervals and sleep in a more awakened state than humans. It can be challenging to tell a sleeping fish from an awake fish. Many researchers refer to fish sleep as “a period of rest.”

Brain Activity and Fish Sleep

Extensive sleep research has been performed on zebrafish. Experts compare the brain waves of sleeping fish to that of other land animals and humans. Research has shown that zebrafish have slow-wave and REM sleep, the same as humans. The fish also slowed their heart rate and reduced muscle tone during rest periods. Sleep-related brain waves are usually recorded in the part of the brain called the neocortex. Fish don’t have a neocortex, but sleep-like brain waves were found in another part of the fish’s brain called the dorsal pallium.

Behavioral Activity and Fish Sleep

Reduced responsiveness to the environment and temporary periods of inactivity define the behavioral definition of sleep in animals. Again, zebrafish were used in studies to learn how fish exhibit behavioral sleep patterns. Many other fish species exhibit the same sleep patterns and behaviors. Zebrafish were found to stop moving for periods of time and were observed to be in a less responsive state. Researchers could easily pick up, move, and handle the fish during these rest periods. Nurse sharks will rest at the bottom of the ocean and become less responsive to the divers around them. When goldfish and perch sleep, they are unresponsive to someone touching their tail. These behaviors all indicate a sleep-like state even when a fish looks awake!

You may notice that fish sleep stages sound familiar to your own. Humans have four stages of sleep, while fish have two. These two stages resemble a human’s slow-wave sleep and REM sleep. Fish also suffer from sleep deprivation. After a night of interrupted sleep, they were found to sleep more. This is called a sleep rebound. Fish were also found to experience microsleeps. Researchers found that when fish were kept awake for long periods that the fish would nod off and attempt to rest whenever possible.

Do Fish Have a Circadian Rhythm?

Most people go to bed around the same time every night and wake up around the same time every morning. This sleep-wake cycle is called a circadian rhythm. Circadian rhythms are 24-hour cycles and keep your internal clock on track. This internal clock is affected by environmental stimuli such as light. This is why most people sleep during the night and are awake during the day. Fish have a circadian rhythm too. They follow regular sleep-wake cycles as humans and other animals do. Another study on zebrafish showed that the fish’s sleep was much more reduced when exposed to light rather than an electrical or mechanical stimulus.

Where do Fish Sleep?

Where a fish sleeps depends on their environment and the type of fish. Some coral reef fish will float in place when they are ready to rest. Other species of coralfish will only sleep under the protection of their reef. Some fish will burrow in the mud or sand to disguise themselves from predators. Nurse sharks will rest on the sand at the bottom of the sea.

Fish breathe the oxygen that gets separated from the water by the fish’s unique gills. This means that water must constantly move over the gills. When a fish stops moving, less water will pass against its gills, which means it will receive less oxygen. To combat this, some fish rest in facing a current to allow water to continue to move across their gills. Other fish species will occasionally be stationary while moving a fin to keep the water moving. Larger fish use what is called unihemispheric sleep. This type of sleep allows a fish to put one-half of its brain to sleep at a time. This allows the fish to continue swimming slowly but enough to keep water moving over their gills. Sharks and stingrays use unihemispheric sleep.

How to Tell if a Fish is Sleeping

Now that you have learned that fish sleep, you may want to know how to tell if your fish are sleeping. Here are some signs that indicate your fish may be asleep or in a period of rest:

• You notice your fish becomes motionless around the same time every day.
• Your fish takes longer to respond to light or food being dropped into its tank.
• It looks to be floating in place.
• It’s motionless at the bottom of its tank.
• Your fish hasn’t moved for a few minutes.