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

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

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

Brain activity during a coma is estimated to be 40% compared to baseline in wakefulness, while in the deepest moment of all sleep there is still brain activity 60%.

On the other hand, in the REM phase of sleep (established within an hour and a half of starting to sleep), brain activity is very similar to that present in a full waking state.

We can take for granted the physiological realities that happen in our body, but the reality is that not even we We know why many of the processes that define us as a species take place, no matter how accustomed we are to taking them to cape.

If you want to know why we sleep, read on.

• Related article: "Circadian rhythms: what are they and in what biological functions do they intervene"

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 circadian rhythms that surround us, a series of oscillations of biological variables in time intervals repeated.

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

When the NSQ perceives that daylight is beginning to decrease (20: 00-22: 00 H), through polysynaptic pathways it promotes the synthesis of melatonin in the pineal gland. The concentration of this hormone induces sleep in humans and its peak of presence in the blood occurs at 2:00 or 3:00 in the morning. 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, furthermore, explains why we are more sleepy at night and receive continuous signals that we should sleep when it goes down Sun. In any case, this mechanism allows us to know how fatigue and the desire to rest are induced, but not explains why this physiological process has been established in the evolution of living beings throughout the weather.

Why do we sleep (and we need to)

To understand the importance of sleep, just go to the beginning of the ockham razor: “All things being equal, the simplest explanation is usually the most likely”. If living beings sleep, it is because it is necessary, that simple. We develop the idea a bit: if rest were an anecdotal adaptation in the animal kingdom, the following postulations should be fulfilled:

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

None of these rules are met. Although there are living beings that are constantly in flight or swimming, it should be noted that many of them achieve it through a unihemispheric dream, that is, thanks to a slow wave brain rest that only occurs in the middle of the brain (the eye opposite the awake hemisphere remains open).

On the other hand, some species of birds do rest both hemispheres at the same time, but in periods of 5 seconds, while they are in the gliding phase of flight. Giraffes, many fish, and other animals also rest on their feet or in motion, for exceptionally short periods. With these data, an idea is clear to us: all neurologically complex animals sleep, in one way or another.

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

• You may be interested in: "The 5 phases of sleep: from slow waves to REM"

The physiological effects of sleep

The act of sleeping is a universal trait and therefore must have some beneficial effect on the beings who practice it. First of all, it should be noted 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 takes time to restore itself.

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

This claim is justified by a very well documented physiological event: cellular metabolism produces reactive oxygen species (ROS), highly reactive very small molecules that damage cell DNA and oxidize polyunsaturated fatty acids, among other harmful 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 produced by your own metabolism.

When the brain is not constantly integrating information, metabolic rates are reduced and, therefore, the production of reactive oxygen species also decreases. Thus, neuronal and accessory cells are less exposed to physiological stress and give them time to recover. You cannot avoid aging and cell damage caused by your own life, 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 the dream really occurs so that we can integrate the information that we have learned during the day. We ask you the following question: why would a fish of a given species (which does not present learned inheritance or constructions complex social problems) also rests, if it does not require consolidating the information learned because it is not even capable of retain her?

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 phasebut not the main reason we sleep **. If this were the case, only animal species with the capacity to learn and retain experiences would sleep.

The dream and the selection

At this point, it should be noted that the forces of natural selection that act on the world's species do not favor longevity for their own sake. If the dream exists it is not to allow the animal to live longer without meaning, but to act as accurately as possible in the lifetime and can be reproduced as much as possible before To die.

For example, in rats, 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 entire species. For this reason, the “no-sleep” trait has never been fixed in populations, despite the fact that there are certain disorders that lead to it. Everything that is maladaptive is discarded in nature.

Resume

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

For this reason, heritable pathologies that prevent sleep (such as fatal familial insomnia) are extremely rare in the general population and do not spread. The people who carry them die and do not reproduce, so the trait does not spread. In short, we sleep because resting delays senescence and allows us (at an evolutionary level) to recover from the metabolic damage generated by the very functioning of cells.