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Do all animals sleep?

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It has always been believed that sleep is a necessary process for the brain to rest and consolidate information, but this belief excludes other simpler organisms, who have no brain, from the idea that they too may present a dream state.

In the following article we will describe how sleep is currently defined, what research on sleep act of sleeping performed with simpler organisms have been carried out and what results have been obtained.

  • Related article: "The 5 phases of sleep: from slow waves to REM"

Do all animals sleep or do they need to sleep?

In various investigations carried out with different animals with simpler structures, without a brain or central nervous system, periods of decreased activity and behavior have been observed, being able to be linked with the sleep phase. In the same way, it has been seen that if these living beings are deprived of sleep, they also presented alterations in their functioning.

Thanks to the evolution of these studies, it is currently considered that these periods of rest and sleep are more related to cellular and molecular processes, and not simply behavioral.

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Now, to know if all animals sleep, we must first ask ourselves what it means to sleep.

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How do you define the dream?

Even today, the definition of sleep as a period of rest of the brain has scientific consensus, but we cannot be left with such a reductionist explanation of the phenomenon of sleeping, since in this way we limit the living beings that can present sleep.

Sleep has been defined as a process of rest and of restoring both physical and psychological functions.. Sleep has been seen to be essential for rest, as well as for learning processes. The brain uses sleep to consolidate memories and eliminate toxic waste. Similarly, it can also be related to the brain plasticity, generating and strengthening some connections between neurons and eliminating others.

It is a period in which there is a temporary loss of consciousness, observing a characteristic type of brain activity while we sleep; eye movements and changes in muscle tone also appear. These changes have been studied using physiological tests such as polysomnography, which allows record brain activity, breathing, heart rate, muscle activity, and levels of oxygen.

Animal sleep

It has been proven that during sleep we go through different phases. The non-REM phase appears first, leading to slow, synchronized brain waves, few eye movements, and a decrease in brain temperature. The REM phase is the one in which waves more similar to those of the waking period appear; more eye movements and muscle atony are observed. Non-REM sleep is considered essential for brain rest and REM was more related to the consolidation of memories and learning.

As we advance at the beginning of the section, there is consensus about the idea that sleep function is important for optimal recovery and functioning. of the brain, but it cannot be the only usefulness of sleeping, since in this way only living beings with brain. Contrary to this reductionist belief, it is known that sleep also involves a change in the behavior of animals without a nervous structure that can be considered a brain, observing a decrease in activity.

It is logical to think that the usefulness of sleep goes beyond meeting the needs of the brain, since in every being I live, by the fact of living, there is wear that will need to be repaired with periods of rest or less exercise.

In this way, It has been seen that smaller and simpler beings, with fewer types of cells, less complex molecules and simpler behaviors can show periods of sleep. Therefore, it is necessary to investigate and verify the change in the behavior of simpler organisms in order to be able to relate them to sleep states.

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Studies that support the presence of sleep in different living beings

If we take into account different ways of measuring and analyzing sleep, such as taking into account physiological and behavioral activity, we see different characteristic patterns in different animals. Animals, such as cows, have been seen to sleep standing up; others who sleep while swimming; and others that have the ability to deactivate the waking state in one cerebral hemisphere while keeping the other hemisphere conscious, as would be the case with dolphins.

It has also been observed that bats sleep 20 hours or that octopuses change color different times while they sleep. Thus, we cannot be guided by a specific behavior when we study sleep in animals that are very different from each other. We must take into account different behaviors that indicate periods of sleep or rest. In this sense, the majority of animals studied with electrical recording techniques have shown to have at least two stages or sleep phase.

Thus, some authors have considered that if you live it is necessary to sleep regardless of the animal species. In this way, studies have been carried out with invertebrate beings such as fruit flies and worms, or even with simpler organisms such as sponges.

If sleep is observed in these living beings, two beliefs would be supported. The first will affirm that sleep is also necessary for the proper functioning of the muscles, the immune system and the intestine, these can also influence how and when the sleep. In the same way, this could mean a change in the study of the functions of sleep in the different processes, ceasing to focus only on the most complex and also assessing how it affects basic cellular functions.

For this reason, research began with less evolved organisms, with ancestral morphological characteristics; In these studies, the researchers were faced with a need to define how to measure sleep or rest in these simpler animals. In this way, they valued when the behavior of these beings decreases and what happens if they are disturbed and they are not allowed to rest.


It was Michael Abrams who in 2017 observed the Cassiopea, a type of jellyfish Characteristics for being most of the time upside down so that the light better reaches the photosynthetic mechanisms and thus be able to have energy. It was found that these movements to obtain energy decrease at night.

In the same way, the test of disturbing the jellyfish at night was also carried out so as not to let them rest, thus observing that the following day they were less active. They were also given during the day melatonin, a hormone linked to the period of sleep, and it could be seen how the activity of the jellyfish decreased to levels similar to those at night.

Hydra vulgaris

The Hydra vulgaris, a tiny animal that, like jellyfish, also does not have a brain. This animal was seen to be less active in the dark. In the same line, hydras deprived of sleep produced changes in the activity of 200 genes, thus signaling the presence of a molecular change. Sleep was no longer defined in these simple species only behaviorally and physiologically, but rather cellularly and molecularly.

Studies have also been carried out with placozoans, possibly the animals with the simplest structures on the planet, which feed on microalgae. In these studies, it has been observed that during the night there was a decrease in the activity of searching for food, which would allow these beings to rest, and thus relating it to one of the first steps towards sleep.

in addition Research was carried out with marine sponges, observing resting cycles that allow them to rejuvenate and reorganize cells. It has also been seen that there are parts of the body that stop pumping water and are associated with sleep-like behaviors in the same way.

Of course, investigations with both placozoans and sponges have presented complications, since these Animals need very specific conditions to live and end up dying quickly, thus making it difficult to study.

From what has been seen in the research, it is intuited that both animals with a nervous system and those that are simpler go through a period of sleep or similar to sleep; it has been suggested that the most complex organisms have not evolved the ability to sleep, but rather we have evolved the ability to be awake.

  • Related article: "Comparative Psychology: the animal part of psychology"

New treatments for sleep disorders

These new insights and discoveries about sleep could help develop new interventions to treat sleep disorders. It has been observed that all cells in the body benefit from sleep, so it makes sense to think that all of these are related to the onset of sleep.

In addition, research with mice has observed the function of a protein that allowed sleep-deprived mice to stay awake. Similarly, it was seen in these animals that the gastrointestinal tract, pancreas and fatty tissue they generated molecules (called neurohormones) that influence the initiation and duration of sleep.

In conclusion, if we learn about new mechanisms, processes or organs in the body that control sleep beyond the brain, new treatments could be tried to decrease sleep problems, as well as detecting other causes that produce alterations in sleeping behavior and conducting research to address them.

This new knowledge also allows us to better understand the effects that sleep deprivation produces, resulting in an effect on the body's state of health and its performance.
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