All animals need to sleep or, at least, enter a resting state where the experiences lived during wakefulness are consolidated and where your body rests. Not all animals sleep the same way or need to sleep the same number of hours.
For example, prey animals, such as hoofed animals, sleep for very short periods of time and can even sleep standing up. Predators, however, can sleep for several hours, they are not always very deep dreams but they are in a state of dormancy, a clear example is the cat.
Animals that live in the water, like fish, also need to enter that sleep state, but how do fish sleep?Because if a fish were to sleep like any land mammal does, it could be swept away by currents and end up being devoured.
To find out, don't miss this article on our site where we will learn what system they use and how the fish sleep. We will also solve questions such as: do the fish sleep at night? or how many hours do the 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 sleeping state and an awake state, is mediated by neuronslocated in a region of the brain called hypothalamus These neurons release a substance called hypocretin and its deficit produces narcolepsy.
In subsequent research it was shown that fish also have this neuronal nucleus, therefore, we can say that fish sleep or, at least they have the tools to do it.
How do fish sleep?
In the first instance, it is difficult to determine sleep in fish In mammals and birds, techniques such as the electroencephalogram are used, but this is linked to the cortex of the brain, a structure that fish lack, moreover, using an encephalogram in an aquatic environment is not feasible.
To recognize sleep in a fish we must pay attention to certain behaviors, such as:
- Long Inactivity. When a fish remains motionless for a long time, at the bottom of a reef for example, it is because it is sleeping.
- Use of shelter. The fish, when they rest, look for a refuge or hidden place to shelter while they sleep. For example, a small cave, a rock, an algae…
- Sensitivity decrease. When sleeping, fish decrease their sensitivity to stimuli, so they will not be reactive to events around them, unless they are very noticeable.
In many cases, fish lower their metabolic rate, slowing their heart rate and breathing rate. For all these reasons, even if we do not see a fish sleeping as we can see in any of our pets, it does not mean that a fish does not sleep.
When do fish sleep?
Another question that could arise when we try to understand how they sleep is when fish sleep. Fish, like many other living beings, can be nocturnal, diurnal or crepuscular animals and, depending on their nature, they will sleep at one time or another.
For example, Mozambique tilapia (Oreochromis mossambicus) sleep at night, sinking to the bottom, slowing their breathing rate and immobilizing their eyes. On the contrary, the brown bullhead fish (Ictalurus nebulosus), are nocturnal animals and spend the day in a refuge with all their fins lax, that is, relaxed. They do not respond to sound or contact stimuli and have a very slow pulse and breathing.
Tench (Tinea tinea) is another nocturnal fish. This animal sleeps during the day, lying on the bottom for periods of 20 minutes In general, fish do not sleep for long periods of time, the cases that have been studied are always a few minutes.
Do fish sleep with their eyes open?
A widespread popular belief is that fish don't sleep because they don't close their eyes. Thinking this is wrong. Fish simply can never close their eyes since they don't have eyelidsFor this reason, fish always sleep with their eyes open
However, some types of sharks have what is known as nictitating membrane or third eyelid, which serves to protect the eyes, although they do not close them to sleep either. Unlike other fish, sharks cannot stop swimming since, due to the type of breathing they do, they need to be in constant movement so that the water passes through the gills and thus be able to breathe. Therefore, while they sleep, the sharks remain in movement, although this is very slow. Their heart rate and breathing slow down, as do their reflexes, but being predatory animals, they needn't worry.
If you want to know how other aquatic animals sleep, don't miss the article How do dolphins sleep?
