Why do we sleep? The biological processes associated with this phenomenon

Why do we sleep to the point of needing to sleep to live? Let's look at its biological functions.

Sleep is defined as a naturally recurring state characterized by altered consciousness, relatively diminished sensory activity, reduced interaction with the environment, and inhibition of the activity of almost all voluntary muscles (during the REM phase).

Sleep is considered to be an essential activity for all animals, as it is established at the level of the human brain.It is considered an essential activity for all animals, as it is evolutionary established in any complex taxon. When we rest, we are in an intermediate situation between wakefulness and total loss of consciousness.

It is estimated that brain activity during a coma is 40% compared to the basal state of wakefulness, while in the deepest moment of sleep, brain activity is still 60%.

On the other hand, in the REM sleep phase (which occurs one and a half hours after the onset of sleep), brain activity is very similar to that present in a fully awake state.

We may take for granted the physiological realities that happen in our body, but the reality is that even we do not know why many of the processes that define us as a species take place, no matter how accustomed we are to carrying them out. If you want to know why we sleep, read on..

Circadian rhythms and the biological clock

Understanding why we sleep is not entirely easy, but the mechanism that causes this situation has been described on multiple occasions. First of all, it should be noted that living beings develop based on the circadian rhythms that surround us, a series of oscillations of biological variables in repeated time intervals.

The biological clock of each organism (located primarily in the hypothalamus, specifically in the suprachiasmatic nucleus NSQ) controls the actions and metabolism of the individual according to the specific moment in each of these circadian rhythms. For example, with light exposure, the NSQ sends inhibitory signals to the pineal gland, which is responsible for synthesizing melatonin from tryptophan (and yielding the intermediate metabolite serotonin).

When the NSQ senses that daylight is beginning to decrease (20:00-22:00 H), through polysynaptic pathways it promotes melatonin synthesis in the pineal gland. The concentration of this hormone induces sleep in humans and its peak presence in the Blood occurs at 2:00 or 3:00 am. The presence of light (or its absence) completely modulates melatonin secretion.

This is the clearest example of how biological clocks are integrated with the circadian rhythm and also explains why we are more sleepy at night and why we receive continuous signals to sleep when the sun goes down. In any case, this mechanism allows us to know how tiredness and the desire to rest are induced, but it does not explain why this physiological process has been established in the evolution of living beings over time.

Why we sleep (and why we need to sleep)

To understand the importance of sleep, it is sufficient to refer to the principle of Ockham's razor: "all things being equal, the simplest explanation is usually the most probable". If living beings sleep, it is because it is necessary, as simple as that. Let us develop the idea a little: if rest were an anecdotal adaptation in the animal kingdom, the following postulations should be fulfilled:

• There should be species that do not sleep at all.
• There should be animals within the same species that do not need to rest after staying awake for a long period of time.
• There should be animals that do not suffer any serious consequences from not sleeping.

None of these rules are fulfilled. Although there are living beings that are constantly flying or swimming, it should be noted that many of them achieve this by means of unihemispheric sleepthat is, thanks to a slow-wave brain rest that only occurs in half of the brain (the eye opposite to the awake hemisphere remains open).

On the other hand, some species of birds do rest both hemispheres at the same time, but for periods of 5 seconds, while in the glide phase of flight. Giraffes, many fish and other animals also rest while standing or moving, for exceptionally short periods. With these data, one idea is clear: all neurologically complex animals sleep, in one way or another.

Thus, we sleep because our ancestors slept, because all vertebrates sleep, and because all vertebrates sleep. all vertebrates sleep, and because sleeping is an adaptive character in the animal kingdom that cannot be discarded or modified.. If we get philosophical, we sleep because we cannot conceive life with a nervous system without the rest it requires.

The physiological effects of sleep

The act of sleeping is a universal trait and, therefore, must have some beneficial effect on the beings that practice it. First of all, it is worth noting that sleep allows the brain to rest, since the body's basal metabolism decreases during rest. The brain consumes about 350 kilocalories every 24 hours simply by existing (20% of the body's energy), so it requires some time to restore itself.

"Sleep is of the brain, by the brain and for the brain." Sleep is explained by the brain, is produced by the brain and is for the brain. (Hobson JA, 2005)

This statement is justified by a well-documented physiological event: cellular metabolism produces reactive oxygen species (ROS), very small highly reactive molecules that damage the cell's DNA and oxidize polyunsaturated fatty acidsamong other damaging mechanisms. There are many antioxidants that more or less prevent this process, but one of the keys to aging at the cellular level is exposure to ROS from its own metabolism.

When the brain is not constantly integrating information, metabolic rates are reduced and, as a result, the production of reactive oxygen species is also reduced.. Thus, neuronal and accessory cells are less exposed to physiological stress and are given time to replenish themselves. It is not possible to avoid aging and cell damage as a result of life itself, but it is possible to delay it by lowering metabolic rates, at least for a significant part of the day.

We tend to have an anthropocentric view of things and, therefore, we believe that sleep really occurs so that we can integrate the information we have learned during the day. We ask you the following question: why does a fish of a given species (which does not present learned inheritance or complex social constructions) also rest, if it does not need to consolidate the learned information because it is not even capable of retaining it?

Based on this question, it only remains to think that the use of sleep to consolidate the information received is an effect derived from the sleep phase, but not the main reason for this.but not the main reason why we sleep**. If this were so, only animal species with the capacity to learn and retain experiences would sleep.

Sleep and selection

At this point, it is worth noting that the forces of natural selection acting on the world's species do not favor longevity for its own sake. If sleep exists, it is not to allow the animal to live longer without meaning, but to allow it to act as accurately as possible in its life span and to be able to reproduce as long as possible before it dies.

For example, in rats, the total absence of sleep is fatal in 100% of cases at 3 weeks. The members of this species that do not sleep are weakened, with slow reflexes, metabolic problems and even ulcers in their tissues. The state of "No-rest" drastically decreases the survival of the animal, and therefore, of the whole animal.and, therefore, of the entire species. For this reason, the "non-sleeping" trait has never been fixed in populations, despite the existence of certain disorders that favor it. Anything that is maladaptive is discarded in nature.

Summary

Thus, we dare to conclude that we sleep by a mere mechanism of biological selection. If a living being does not sleep, it dies, does not reproduce and the species becomes extinct, so heritable traits that favor balanced sleep in living beings will always be favored.

Therefore, heritable pathologies that prevent sleep (such as familial fatal insomnia) are extremely rare in the general population and do not spread. People who carry them die and do not reproduce, so the trait does not spread. In short, we sleep because rest delays senescence and allows us (at the evolutionary level) to recover from the metabolic damage generated by cell functioning itself.