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- Do fish sleep? Transition between sleep and wakefulness
- Sleeping fish: signs
- When does the fish sleep?
- Animal that sleeps with its eyes open: the fish
All animals need to sleep or at least enter a rest state that allows to consolidate the experiences lived during the waking period and that the body can rest. Not all animals sleep the same way, nor do they need to sleep the same number of hours.
For example, prey animals, like hoofed animals, sleep for very short periods of time and can even sleep standing up. Predators, however, can sleep for several hours. They don't always sleep very deeply, but they are definitely in a sleep state, as is the case with cats.
Animals that live in water, such as fish, also need to enter this state of sleep, but how fish sleep? Keep in mind that if a fish slept like terrestrial mammals do, it could be dragged by the currents and end up being eaten. To find out more about how fish sleep, don't miss this PeritoAnimal article, as we'll explain what system fish use and how they sleep. In addition, we will address issues such as whether the fish sleep at night or how many hours a fish sleeps.
Do fish sleep? Transition between sleep and wakefulness
A few years ago, it was shown that the passage between sleep and wakefulness, that is, between a sleep state and awake one, is mediated by neurons located in a brain region called hypothalamus. These neurons release a substance called hypocretin and its deficit produces narcolepsy.
In later research, it was shown that fish also have this neuronal nucleus, so we can say that the fish sleep or that they at least have the tools to do it.
Sleeping fish: signs
First of all, it is difficult to determine sleep in fish. In mammals and birds, techniques such as the electroencephalogram are used, but these are related to the cortex of the brain, a structure that is absent in fish. Also, performing an encephalogram in an aquatic environment is not feasible. To recognize if fish sleep, it is necessary to pay attention to certain behaviors, such as:
- Prolonged inactivity. When a fish remains immobile for a long time, at the bottom of a reef, for example, it is because it is sleeping.
- Use of the refuge. The fish, when resting, seek some refuge or hidden place to protect themselves while they sleep. For example, a small cave, a rock, some seaweed, among others.
- Decreased sensitivity. When they sleep, fish decrease their sensitivity to stimuli, so they don't react to events happening around them unless they are very noticeable.
In many cases, fish lower their metabolic rate, lowering their heart rate and breathing. For all this, even though we can't see a sleeping fish as we see other pets, that doesn't mean the fish doesn't sleep.
When does the fish sleep?
Another question that can arise when trying to understand how fish sleep is when they perform this activity. Fish, like many other living things, can be animals night, day or twilight and, depending on nature, they will fall asleep at one time or another.
For example, Mozambican tilapia (Oreochromis mossambicus) sleeps during the night, descending to the bottom, decreasing his breathing rate and immobilizing his eyes. On the contrary, the brown-headed catfish (Ictalurus nebulosus) are nocturnal animals and spend the day in a shelter with all their fins loose, that is, relaxed. They do not respond to sound or contact stimuli and their pulse and breathing become very slow.
The tench (tinea tinea) is another night fish. This animal sleeps during the day, remaining at the bottom during 20 minute periods. In general, fish do not sleep for long periods of time, the cases that have been studied always last a few minutes.
Also check out how fish reproduce in this PeritoAnimal article.
Animal that sleeps with its eyes open: the fish
A widespread popular belief is that fish do not sleep because they never close their eyes. That thought is wrong. Fish just can't ever close their eyes because don't have eyelids. For this reason, the fish always sleep with their eyes open.
However, some types of shark have what is known as nictitating membrane or third eyelid, which serves to protect the eyes, although these animals also do not close them to sleep. Unlike other fish, sharks cannot stop swimming because the type of breathing they do requires that they be in constant motion so that water can pass through the gills so they can breathe. Therefore, while they sleep, sharks remain in motion, albeit extremely slow. Their heart rate and respiratory rate decrease, as do their reflexes, but being predatory animals, they don't need to worry.
If you want to know more about aquatic animals, check out this article by PeritoAnimal about how dolphins communicate.
If you want to read more articles similar to Fish sleep? explanation and examples, we recommend that you enter our Curiosities section of the animal world.