Do all animals sleep?

Have we evolved sleep, or wakefulness? Do all animals need to sleep?

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, which do not have brains, from the idea that they too can present a state of sleep.This belief excludes other simpler organisms, which do not have brains, from the idea that they too can be in a state of sleep.

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

Do all animals sleep or have the need to sleep?

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

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

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

How is sleep defined?

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

Sleep has been defined as a process of resting and restoring both physical and psychological functions.. It has been shown that sleep is 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 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 sleeping; eye movements and change in muscle tone also appear. These changes have been studied by means of physiological tests such as polysomnography, which allows us to record brain activity, respiratory activity, heart rate, muscle activity and oxygen levels.

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

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

It is logical to think that the usefulness of sleep goes beyond meeting the needs of the brain, since in every living being, by the fact of living, there is a wear and tear that will be necessary to repair with periods of rest or less activity.

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

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. It has been observed that animals, such as cows, sleep standing up; others sleep while swimming; and others have the ability to deactivate the state of wakefulness in one brain hemisphere while keeping the other hemisphere conscious, as in the case of dolphins.

It has also been observed that bats sleep for 20 hours or that octopuses change color several times during sleep. Therefore, we cannot be guided by a specific behavior, we cannot be guided by a specific behavior when we study sleep in animals that are very different from one another.. We must take into account different behaviors that signal periods of sleep or rest. In this sense, most animals studied with electrical recording techniques have been shown to have at least two sleep stages or sleep phases.

Thus, some authors have considered that if one lives, it is necessary to sleep regardless of the animal species. Thus, studies have been carried out with invertebrates 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 would assert that sleep is also necessary for the proper functioning of muscles, the immune system and the intestine, and these could also influence how and when sleep appears. Similarly, this could mean a change in the study of the functions of sleep in different processes, We stopped focusing only on the most complex and also assessing how it affects the basic cellular functions of the body..

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

It was Michael Abrams who, in 2017, observed the Cassiopeaa type of jellyfish characteristic for being most of the time upside down so that the light better reaches the photosynthetic mechanisms and so they can have energy. It was verified how these movements to obtain energy decrease at night.

Similarly, the jellyfish were also tested by disturbing them during the night so as not to let them rest, thus observing that the following day they were less active. Melatonin, a hormone linked to the sleep period, was also administered to the jellyfish during the day, and it could be seen how the activity of the jellyfish decreased to levels similar to those of the night.

Also studied was the Hydra vulgarisThe tiny animal, which, like jellyfish, also has no brain. It was seen that, in the dark, this animal was less active. Along the same lines, hydras that were deprived of sleep produced changes in the activity of 200 genes, thus pointing to the presence of a molecular change.. Sleep in these simple species was no longer defined only behaviorally and physiologically, but rather cellularly and molecularly.

Studies have also been conducted 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 foraging activity, which would allow these beings to rest, thus relating it to one of the first steps to sleep.

Likewise, research was carried out with marine sponges, observing cycles of rest that allow them to rejuvenate and reorganize cells. It has also been observed that there are parts of the body that stop pumping water, which is also linked to behaviors similar to sleep.

However, research with both placozoans and sponges has presented complications, since these animals need very specific conditions to live and end up dying quickly, thus hindering the study.

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

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 sleep onset.

In addition, research in mice has shown 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 fat tissue generated molecules (called neurohormones) that influence the onset and duration of sleep.

In conclusion, if we know new mechanisms, processes or organs of the body that control sleep beyond the brain, new treatments could be tested to reduce sleep problems, as well as to detect other causes of sleep disorders and conduct research to address them.We can also detect other causes of sleep disturbances and conduct research to address them.

This new knowledge also allows a better understanding of the effects that sleep deprivation produces, leading to an affectation of the body's state of health and its performance.