Do fish ever sleep? It seems as though they are constantly swimming. Well, fish need sleep just like every other animal on earth. They do this by slowing down their movements until they are stagnant or resting. 

Fish sleep by entering what we humans would refer to as a daydream-type state. They do not close their eyes when sleeping because they do not have any eyelids. If you witness a fish sleeping, you will most likely see the fish hovering in place. They usually sleep toward the bottom of the water column or the bottom of a tank.

The fish are never completely still, however. Most fish need to move slightly to maintain water flow across their gills. This is how the fish regulate oxygen flow. It will be more noticeable with larger fish, as they need more oxygen than smaller fish.

Do Fish Sleep at Night?

Sleeping with their eyes open may seem odd. Because fish do not have to protect their eyes from dust while they are underwater, they do not need eyelids. As far as light cycles go, some fish are diurnal, meaning they are active in the daylight. This is so they can use their eyesight for hunting or seeing food clearly, and sleep during the dark.

Other species are nocturnal. This means they prefer to be active in the dark, as they rely more on their sense of smell to hunt. Fish sleep by drastically slowing their metabolisms and activity level. Also to rest and rejuvenate the body. They do not sleep lying down, but species that live near underwater structures will hide within caves or crevices for safety while sleeping. When sleeping, fish are still coherent enough when sleeping to react to danger or predators.

Why Do Fish Sleep?

Research shows that fish sleep, and need sleep like most other animals. A study carried out by the University of Zurich, evaluated animal sleep. It determined that fish sleep by entering a resting state. This slowed metabolic and heartrate, proving that they do have a sleep cycle. The definition of sleep states that animals, to qualify as sleeping, need to have their eyes closed and the neo-cortex area of their brains shut down, fish do not (in most cases) have either of these features, therefore are not recognized. 

What if Fish Don’t Sleep?

Fish suffer from sleep deprivation just as humans or other animals would if they do not maintain regular sleep patterns. Fish do not enter rapid eye movement sleep like humans or other mammals. But, they still require a regular sleep cycle to function properly. Fish who are tormented during their sleep, either by predators or humans, have been known to catch up on their sleep as soon as the tormenting has ended. 

What Stops a Fish From Sleeping?

Fish do not sleep while spawning when they are in the midst of a migration. They also stay awake when they are caring for their young. Some fish do not sleep at all until they have reached adulthood. Fish, like Bluefin Tuna, never seems to sleep.

Scientists believe that this is because the scenery in which they live does not change very much. Because of this, they do not necessarily need to slow down and process it as often. Other researchers believe that because these fish tend to swim in schools. So, a few fish are designated to watch for food and danger, as the other fish are sleeping. This would occur on a rotating basis. 

Do Fish Sleep in Aquariums?

Fish in an aquarium are the easiest to witness sleeping. These fish end up adopting a schedule that is determined by when the lights are on or off. The fish being fed and not having to hunt for prey as fish would in the wild. Here, it is easy to recognize when the fish are sleeping and their sleeping habits.

Aquarium fish will nestle themselves into the rocks or stones at the bottom of the tank. Once the lights turn off they stay resting until the lights come on again. During this time the fish will barely move at all, just enough to keep balanced for long periods.

How to Spot Sleeping Fish

Next time you find yourself at the fish tank, notice where the fish are and what they are doing. Are they feeding, how are they oriented in the water column, or if they are seemingly motionless. If they are towards the bottom and just moving subtly, you can bet that they are in as close to a sleeping state as fish ever get! Be sure not to wake them though, the startle could cause extreme stress and could end up harming the fish.

John Zahra
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