Do Fish Sleep? | Sleeping Habits of Animals

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Many kids ask whether fish sleep. The short answer is yes. All life forms need to sleep or, at least enter a state of rest. It is important because sleep the experiences lived during wakefulness are consolidated and the body rests. The long answer mainly depends on the definition of sleep.

Not all animals sleep in the same way or need to sleep the same number of hours. For example, prey animals sleep for very short periods of time and can even sleep standing up. Predators, however, can sleep for several hours. They do not always enter deep sleep phase during this time, but they are in a state of dozing. A clear example is the cat who spends up to 17 hours a day sleeping.

Other species, like fish, also need to enter a resting state. However, if a fish were to sleep like any land mammal does, it could be swept away by currents and end up being eaten. Yet, some fish species become so deeply asleep that you can lift them out of the water without waking up at all. So, how do fish sleep? In this article we will answer a few interesting questions about the sleeping behavior of fish. So, if you want to find out whether fish sleep at night or how many hours this specie spends resting, keep reading.

Transition Between Sleep and Wakefulness

Researchers established that the transition between sleep and wakefulness, that is, between a sleeping state and an awake state, is mediated by neurons. These neural networks are located in a region of the brain called the [hypothalamus]. These neurons release a substance called hypocretin and its deficit produces narcolepsy.).

In subsequent scientific research zoology researchers demonstrated that fish also posses this neural nucleus. Therefore, we can say that fish sleep or, at least, are equipped to do so.

How Do Fish Sleep?

It is very difficult to determine whether a fish is asleep or awake. In mammals and birds, scientists use various techniques such as the electroencephalogram (EEG). However, these techniques record the brain’s [cerebral cortex]. activity, a structure that fish lack. Apart form that, using an EEG in an aquatic environment is not feasible. Behavioral science is more appropriate when it comes to studying fish sleeping patterns. To recognize a sleeping fish we must pay attention to certain behaviors:

  1. Prolonged inactivity: When a fish remains immobile for a long time it is because it is sleeping.
  2. Use of shelter: Fish look for a refuge or hidden place to take shelter in while they sleep. For example, a small cave, a rock or algae.
  3. Decreased sensitivity: When sleeping, fish decrease their sensitivity to stimuli, so they are not reactive to events around them, unless they are very noticeable.

In many cases, fish lower their metabolic rate, by decreasing their heart and respiratory rates. Therefore, even if the fish does not display sleeping behaviors similar to other pets, it does not mean that fish don’t sleep.

When Do Fish Sleep?

Another question that could arise when we try to understand how fish sleep is when they sleep. Fish, like many other species, can be nocturnal (active at night), diurnal (active during day time) or crepuscular (active during twilight) animals. Depending on the fish species, they all sleep at one point or another.

For example, Mozambique tilapia (Oreochromis mossambicus) sleep at night. For this purpose they sink to the bottom, slow down their respiratory rate and immobilize their eyes. On the contrary, brown bullhead (Ameiurus nebulosus) fish, are nocturnal animals and spend the day in a shelter with their fins relaxed. They do not respond to sound or stimuli and have a very slow pulse and breathing.

Tench (Tinea tinea) is another nocturnal fish specie. 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 notion is that fish do not sleep because they do not close their eyes. We base this notion on our definition of sleep. However, this conclusion is wrong. Fish can never close their eyes because they lack eyelids. For this reason, if you look at your aquarium you will notice that fish always sleep with their eyes open.

However, some types of sharks have a nictitating membrane or third eyelid. It serves to protect the eyes, although they do not close them to sleep either. Unlike certain species of fish, sharks cannot stop swimming due to the type of breathing they perform. They need to be in constant motion so that the water passes through the gills and thus be able to breathe. For this reason, while they sleep, sharks remain in motion, although it is very slow. Their heart and respiratory rates decrease, as do their reflexes. However, since sharks are predatory animals, they do not have to worry about being hunted during sleep.

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