It has always been believed that sleep is a necessary process for the brain to rest and consolidate information, but this belief rules out other, simpler organisms., who do not have a brain, of the idea that they too can have a dream state.
In the next article, we’ll describe how sleep is currently defined, what research has been done on the act of sleeping with simpler organisms, and what results have been achieved.
Do all animals sleep or do they need to sleep?
In several studies 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. In the same way, we have 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 currently considered that these periods of rest and sleep are more related to cellular and molecular processes, and not just behavioral.
However, to know if all animals sleep, we must first ask what it means to sleep.
How is the dream defined?
Even today, the definition of sleep as a period of rest of the brain is the subject of a scientific consensus, but we cannot stop at such a reductionist explanation of the phenomenon of sleep, because this limits the living beings who can sleep.
Sleep has been defined as a process of resting and restoring physical and psychological functions. Sleep has been shown to be essential for rest and learning processes. The brain uses sleep to consolidate memories and eliminate toxic waste. Likewise, it can also be linked to brain plasticity, generating and strengthening certain connections between neurons and eliminating others.
This is a period in which there is a temporary loss of consciousness and a characteristic type of brain activity is observed while we are sleeping; eye movements and a change in muscle tone also appear. These changes have been studied using physiological tests such as polysomnography, which records brain activity, respiratory activity, heart rate, muscle activity and oxygen levels.
We have discovered that during sleep we go through different phases. The non-REM phase appears early, resulting in slow, synchronized brain waves, little eye movement, and a decrease in brain temperature. The REM phase is the one in which the waves appear more similar to those of the waking period; there is more eye movement and muscle weakness. Non-REM sleep is considered essential for brain rest, and REM sleep is more closely related to memory consolidation and learning.
As we argued at the beginning of the section, there is a consensus that sleep function is important for recovery and optimal brain function, but this may not be the only use of the brain. sleep, because only living beings with a brain would have periods of sleep. . Contrary to this reductionist belief, we know that sleep also involves a change in the behavior of animals without a nerve structure which can be thought of as a brain, a decrease in activity was observed.
It is logical to think that the usefulness of sleep goes beyond satisfying the needs of the brain, because in every living being, as a result of life, there is wear and tear that must be repaired with periods of rest or less activity.
This way, Smaller, simpler beings with fewer cell types, less complex molecules, and simpler behaviors have been shown to exhibit periods of sleep.. Therefore, it is necessary to study and verify the behavioral change of simpler organisms in order to be able to relate them to sleep states.
Studies that support the presence of sleep in different living things
If we consider different ways of measuring and analyzing sleep, such as physiological activity and behavior, we see different characteristic patterns in different animals. We have seen what animals, like cows, sleep upright; others who sleep while swimming; and others which have the ability to deactivate the wakefulness 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 several times during their sleep. For that, we cannot be guided by specific behavior specifically 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, it has been shown that most animals studied with electrical recording techniques have at least two stages or phases of sleep.
So, some authors have believed that if you live, you must 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 were observed in these living things, two beliefs would be held. The first will clarify that sleep is also necessary for the proper functioning of muscles, the immune system and the gut, and these can also influence how and when sleep occurs. Likewise, it could lead to a change in the study of sleep functions in different processes, letting us focus only on the more complex and also valuing how this affects basic cellular functions.
This is why research began with less evolved organisms, with ancestral morphological characteristics; in these studies, the researchers found 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 Cassiopeia, a type of jellyfish characteristics to be most of the time upside down so that the light reaches the photosynthetic mechanisms better and thus be able to have energy. We have seen how these movements to obtain energy decrease at night.
Likewise, the test was also carried out to disturb the jellyfish during the night so as not to let them rest, thus observing that the next day they were less active. They were also given melatonin during the day, a hormone linked to the period of sleep, and we could see how jellyfish activity decreased to levels similar to those at night.
Hydra vulgaris, a small animal that, like jellyfish, does not have a brain, has also been studied. We have seen that, in the dark, this animal was less active. In the same vein, in sleep-deprived hydrates, changes in the activity of 200 genes occurred, indicating the presence of a molecular change.. Sleep was no longer defined in these simple species only in a behavioral and physiological way, but rather in a cellular and molecular way.
Studies have also been done with placozoa, perhaps the most structurally sound animals on the planet, which feed on microalgae. In these studies, it was observed that during the night there was a decrease in foraging activity, which would allow these beings to rest, and thus connect it to one of the early stages of sleep.
the same research has been carried out with marine sponges observing rest cycles that allow them to rejuvenate and reorganize cells. We have also seen that there are parts of the body that stop pumping water by similarly associating with sleep behaviors.
However, research on placozoa and sponges has presented complications, as these animals need very specific conditions to live and end up dying quickly, making them difficult to study.
From what has been seen in research, it is believed that animals with a nervous system and the simplest go through a period of sleep or sleep-like; it has been suggested that more complex organisms, we have not developed the ability to sleep, but rather have developed the ability to stay awake.
New treatments for sleep disorders
This 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 is logical to think that all of these cells are related to the onset of sleep.
What’s more, research on mice observed the function of a protein that allowed sleep-deprived mice to stay awake. Likewise, in these animals, it has been observed that the gastrointestinal tract, pancreas and adipose tissue generate molecules (called neurohormones) that influence the onset and duration of sleep.
In conclusion, if we know of any new mechanisms, processes or organs in the body that control sleep beyond the brain, new treatments could be tried to reduce sleep problems, as well as to detect other causes that cause alterations in sleep behavior and to conduct research to address them.
This new knowledge also allows us to better understand the effects of sleep deprivation, leading to an impact on the health and performance of the body.
- Pennisi, E. (2021) The simplest of dreams. Evidence from ancient evolutionary creatures reveals that sleep isn’t just for the brain.
- Dream Institute. What is a dream? Madrid, Spain.
- Roth, T. (2007). Insomnia: definition, prevalence, etiology and consequences. Journal of Clinical Sleep Medicine, 3 (5 Suppl): S7-S10.