Do Fish Sleep?

We all know the importance of deep sleep on our health and cognitive performance. But do you know that other animals also sleep, including fish?

Whether fishes sleep or not, Its answer implies a deep and surprising dive into aquatic biology. Unravel the mystery by reading further! 

Do Fish Sleep?

Yes, fishes sleep, but it is so different from humans and other animals that researchers prefer to call it ‘rest.’ A fish’s sleep does not exhibit the brain wave patterns shown during human sleep and that of many animals.  

Another unique feature is that fish sleep with their eyes open, mainly because they do not have eyelids. They are also more alert than humans in their sleep and react faster to any potential threat. But their metabolic rate slows down like humans. They also slow down physically, and some just float. 

Unlike humans, fish show two stages of sleep (slow-wave and REM sleep) and even the ill effects of sleep deprivation.  

Do All Fish Sleep?

Almost all species of fish sleep, apart from a few exceptions.  

Few species, like the blind, cave-dwelling fish and the deep-water fishes forego sleep and swim continuously.  

The explanation lies in the theory that fish sleep to process the visual stimuli from the day. But cave-dwellers have no sight, and deep swimmers have negligible to no information to process. Hence, they do not need sleep.  

How Do Fish Sleep?

We have already mentioned that fish sleep with their eyes open. However, they also remain in motion, which isn’t aggressive swimming but just floating.  

They occasionally move their fins to stay balanced and keep the oxygen supply going and keep their balance. However, they barely show any other movement apart from this subtle movement. Due to this, they are primarily unable to swim at their normal elevation and tend to drift down to the bottom of the surface.  

Most fishes are diurnal, i.e., they remain awake during the day and sleep at night. However, a few species are nocturnal. A few other species prefer hiding in a safe place to escape a predator attack.

Do Fish Need Darkness to Sleep?

While fishes do not need complete darkness like humans to sleep, they get better rest in the degree of darkness that is available in their natural habitat.  

While fishes sleep in light, it only happens when they are tired. Moreover, sleep in the light is not restful enough for the fish’s health. Light stresses the fish.  

Hence, it’s best to let the fish sleep in darkness.  

Where do Fish Sleep?

The way a fish sleeps in the ocean and other water bodies is different from the way fish would sleep in a tank. Let’s find out: 

How do Fish Sleep in The Ocean?

The fish species decide the location of a fish’s sleep. For instance, a coral fish stops moving and floats in a place while sleeping. On the other hand, the Nurse sharks sleep along the rocky bottom or near a rocky cave. 

Few species of fish add an extra barrier of protection while sleeping. For instance, certain species of coral fish retreat within the reef to sleep. Parrot fish can create a protective mucus cocoon when sleeping under the coral. Fish like rainbow wrasse usually burrows into sand or mud to sleep. Similarly, sharks tend to sleep in a group.  

Fishes sleep on their sides at the bottom of the sea bed. But fishes like loaches sleep upside down.  

How Do Fish Sleep in a Tank?

When kept in a tank, fishes prefer to sleep near the water’s middle or bottom column. They can sleep 8-12 hours and remain active all day.

The Stages of Sleep in Fish

Like humans, fish also have cycles of sleep. Their body, too, produces hormones that regulate their sleep patterns and also calibrate their internal body clocks. 

As already stated, fishes do not show four stages of sleep like humans but showcase two stages of sleep.

The first stage is that of slow wave, deep sleep. In this stage, the fish’s brain produces a series of synchronized, low-frequency brain waves that help recuperate their bodies.  

Like humans, the second stage is the REM or Rapid Eye Movement Stage. The only difference is that, unlike humans, their eyes do not move rapidly even though their brains are bursting with activity like humans. However, it does not imply that fishes also dream like us. Researchers are still investigating the subject.  

Do Fish Have a Circadian Rhythm?

Fishes, like Zebrafish, have a circadian rhythm, as seen by their regular sleep-wake cycle, just like humans. However, their rhythm is also sensitive to light; hence, fishes are also sensitive to light-dark cycles.  

Fishes like zebrafish also show insomnia and sleep deprivation ill-effects when they do not rest adequately. If exposed to bright light, fishes miss out on sleep and take a long sleep the next day to rejuvenate them.  

However, fishes do not adhere strictly to their circadian rhythm. For instance, species of fish like the white sucker fish become nocturnal instead of diurnal when separated from their school. They easily adjust their circadian rhythm according to their environment.  

Estivation Vs Sleep

Fishes also demonstrate estivation or long summer sleep, which differs from their daily sleep or torpor.  

In the too-hot and dry summer months, many fishes become dormant. It is characterized by inactivity and lower metabolic rate, just like inertia. However, while sleep or torpor can be between 9-14 hours and always less than 24 hours, the estivation continues for about two to four months to survive any dry season. 

A fish’s sleep has two stages, but their estivation has three phases:- induction, maintenance, and arousal. 

Conclusion

Whether do fishes sleep or not is a matter of extreme curiosity to scientists, not only to understand their correlation to humans but also to understand human sleep better.  

We hope you will now better understand your fish sleeping in the aquarium and will make things more comfortable for the swimming creature. 

FAQs