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 can present a dream state.
In the following article we will describe how sleep is currently defined, what research on the act of sleeping carried out with simpler organisms has been carried out and what results have been obtained.
In various investigations carried out with different animals with simpler structures, without a brain or central nervous system, periods have been observed in which there is a decrease in activity and behavior, which can 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 had 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.
Now, to know if all animals sleep, we must first ask ourselves what it means to sleep.
How is sleep defined?
Even today, the definition of sleep as a period of rest for the brain has scientific consensus, but we cannot stop with such a reductionist explanation of the phenomenon of sleeping, since in this way we limit the living beings that can experience sleep.
Sleep has been defined as a process of rest and reestablishing both physical and psychological functions It has been seen that sleep is essential for rest, as well as for learning processes. The brain uses sleep to consolidate memories and eliminate toxic waste. In the same way, it can also be related to brain plasticity, generating and strengthening some connections between neurons and eliminating others.
It is a period in which a temporary loss of consciousness occurs, observing a characteristic type of brain activity while we sleep; Eye movements and changes in muscle tone also appear. These changes have been studied through physiological tests such as polysomnography, which allows recording 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, giving rise to slow and synchronized brain waves, few eye movements and a decrease in brain temperature. The REM phase is the one in which waves appear most similar to those of the waking period ; 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 mentioned at the beginning of the section, there is consensus about the idea that the function of sleep is important for optimal recovery and functioning of the brain, but it cannot be the only usefulness that the fact of sleep has, since in this way it only Living beings with brains would carry out periods of sleep. 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 living being, due to the fact of living, wear and tear occurs that will need to be repaired with periods of rest or less activity.
Thus, 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 to be able 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 seen that animals, such as cows, that sleep standing up; others who sleep while swimming; and others that have the ability to deactivate the state of wakefulness in one cerebral hemisphere while keeping the other hemisphere conscious, as would be the case of dolphins.
It has also been observed that bats sleep for 20 hours or that octopuses change color several 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 signal periods of sleep or rest. In this sense, the majority of animals studied with electrical recording techniques have been shown to have at least two stages or phases of sleep.
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 state that sleep is also necessary for the proper functioning of the muscles, the immune system and the intestine, and these can also influence how and when sleep appears. Likewise, this could mean a change in the study of the functions of sleep in the different processes, stopping focusing only on the most complex ones 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, researchers found themselves needing 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 Cassiopeiaa type of jellyfish characteristics for being upside down most of the time so that light reaches the photosynthetic mechanisms better and thus has energy. It was found how these movements to obtain energy decrease at night.
Similarly, a test was also carried out of disturbing the jellyfish during the night to not let them rest, thus observing that the next day they were less active. Melatonin, a hormone linked to the sleep period, was also administered during the day, and it was possible to see how the activity of the jellyfish decreased to levels similar to those at night.
It has also been studied Hydra vulgaris, a tiny animal that, like jellyfish, also has no brain. It was seen that, in the dark, this animal was less active. In the same line, Hydras that were deprived of sleep showed changes in the activity of 200 genes, thus indicating the presence of a molecular change Sleep in these simple species was no longer defined only in a behavioral and physiological way, but rather in a cellular and molecular way.
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 foraging activity, which would allow these beings to rest, thus relating it to one of the first steps towards sleep.
In addition Research was carried out with marine sponges, observing rest cycles that allow them to rejuvenate and reorganize their cells It has also been seen that there are parts of the body that stop pumping water, linking in the same way with behaviors similar to sleep.
Of course, research with both placozoans and sponges has presented complications, since these animals need very specific conditions to live and end up dying quickly, thus making the study difficult.
From what has been seen in research, it is understood that both animals with a nervous system and those with simpler systems go through a period of sleep or sleep-analogous; It has been suggested that Most complex organisms have not evolved the ability to sleep, but rather we have evolved the ability to be awake
New treatments for sleep disorders
These new knowledge 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.
Furthermore, research with mice has observed the function of a protein that allowed sleep-deprived mice to stay awake. Likewise, it was seen in these animals that the gastrointestinal tract, pancreas and fatty tissue generated molecules (called neurohormones) that influence the onset and duration of sleep.
In conclusion, If we know new mechanisms, processes or organs in the body that control sleep beyond the brain, new treatments could be tested to reduce 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 of sleep deprivation, leading to an impact on the body’s health and performance.
