Do fish really sleep? It’s a question that puzzles many, especially when observing their seemingly constant movement. Without eyelids and the traditional signs of sleep, how can we tell?
The answer is yes, they do.
However, fish have a completely different way of sleeping than land animals, reflecting their aquatic habitat. Fishes do not have eyelids and often do not sleep like REM cycles as mammals do. Instead of lying down, fishes decrease their activity and metabolism while remaining semi-aware in response to predators.
Several fishes take rest by floating at a place, while others find some secure places or burrow themselves into sand and gravel. Sleeping fish do not exhibit the same signs as sleeping land animals; instead, they maintain a state of low-energy respite that allows them to respond rapidly in case of danger.
Different Types of Fish Sleep Behaviors
Fish exhibit a variety of sleep behaviors depending on their species and environment. Some fish enter a state of reduced activity, where they hover in place, barely moving, while others seek refuge in hidden spots like crevices or burrows.
Pelagic fish, which live in open water, often engage in “drift” sleeping, maintaining minimal movement to stay buoyant.
Additionally, nocturnal fish rest during the day, while diurnal fish rest at night. Each behavior reflects an adaptation to their unique ecological niche.
How Do Fish Sleep?
Fish do not sleep like mammals; instead, they exhibit unique behaviors enabling them to sleep while still keeping themselves aware of possible dangers. Unlike humans, fish do not have eyelids and thus they sleep with eyes open, making it hard to tell when they are asleep. Nevertheless, this is only possible by certain body positions and actions.
Body Posture During Sleep
When fish sleep, they often hang in place, swim near the bottom or find a suitable __cpLocation within their habitat. For example, some species lie on the substrate or hide among rocks or plants while others like reef fish anchor themselves into crevices for safety reasons. In the night parrotfish secrete mucus that envelopes them forming a protective cocoon against predators as well as parasites.
Reduced Activity and Alertness
Sleeping fish display reduced activity levels and decrease their respiration rate. Many fishes however can partially remain conscious even when resting to be quick in responding to danger. This lowered state of activity helps conserve energy and reduces risk of predation. For example schooling fishes may continue swimming together while sleeping thus retaining some movement as a defense mechanism against predation so as to stay together and reduce vulnerability to it.
Unihemispheric Slow-Wave Sleep
Certain types of sharks exhibit unihemispheric slow-wave sleep (USWS), which causes one side of the brain to sleep while the other stays awake. This ensures that the fish keeps swimming continually and maintains itself at a specific position in the water column or continues breathing through movement.
All in all, fishes are known to sleep with reduced activity, certain postures, and partial waking. These changes enable them to rest yet retain their capability to act in response to threats from their surroundings; an essential harmony for life’s survival within the aquatic environment.
When do fish go to sleep?
When fish sleep has a lot to do with how they behave and where they live in the ecosystem. Fish that are busy during the day usually rest at night to recover the energy they use during the day.
Nocturnal species, on the other hand, like cardinalfish, are busier at night and rest during the day so they are less likely to be eaten by predators and can eat more when it is dark. But generally, the rule for above water is the same as the rule for below water, the Sun set on the surface will cause the same light loss under the surface and each species will begin its sleep or wake cycle.
How long does a fish sleep?
Depending on the environmental surroundings, the length and frequency of sleep varies greatly between species. And this has been proven by scientific research. Factors effecting this are availability of food, environmental temperatures, interactions with other organisms and of course what the risk is of being eaten by something else in your habitat. Some fish may rest for short amounts of time here and there during the day, while others sleep for longer periods at night.
Just like you and your partner really, they may wake up and look at you with envy as you have had a full night’s sleep while they have been restless all night!
Read Also: Interesting Things to Know About Seahorses
How Can You Tell If a Fish Is Sleeping?
To spot a sleeping fish, you must pay close attention to its behaviour. Some signs are moving less, reacting more slowly to things that happen around them, and looking for safety in places like coral forms or underwater structures.
Keeping an eye on these behaviours teaches us a lot about fish biology, like how they adapt and how they connect with others in complex marine environments. On a night dive, you may spot a small patch of colour that is out of place, if you look closer you can often see that it will be a fishtail protruding from underneath a coral where they have lodged themself and are attempting to stay still to avoid any predator’s attention.
If you want to learn more about fish sleep, one of the best ways is to jump in the water. A night dive in underwater world around Indonesia is quite possibly one of the best places in the world to do this.
There are intriguing things to discover like how reef dwellers have changed their behaviour or how deep-sea dwellers have adapted to their environment, scuba diving can open your eyes to a whole new waking and sleeping world of underwater wonder.
