Why sleep?

2005/07/01 Agirre Ruiz de Arkaute, Aitziber - Elhuyar Zientzia Iturria: Elhuyar aldizkaria

• Anatomia/Fisiologia

Neurobiologists, by keeping lab rats awake, see how their health gets worse quickly. They start to lose weight, lose the ability to maintain body temperature, the immune system gives problems and increases infections. In the end, 20 days rats die from insomnia, the same time they need to starve.

Sleeping is therefore not a whim of the body; the organism is essential to survive. Neuroscientists, physiologists, zoologists and psychologists have been studying the true function of sleep for fifty years and have not yet resolved it.

The doubt is that sleep is for the brain. Animals sleep, but not plants. And the other organs of the animals do not have “sleep” either: neither lungs, nor liver, nor other organs.

According to some researchers, the basic function of sleep is to recover energy lost during the day. The sleep period is a time of low metabolic demand and can be used by neurons to solve the energy shortage caused by intense daily activity. In cells, the energy is stored in the ATP molecule, and as energy is spent, the adenosine molecules that break and break away from it begin to accumulate outside the cell. Neurons take advantage of the time of sleep to reintroduce this adenosine into the cell and recover ATP reserves.

But while we sleep, the metabolic rate is only reduced by 15% compared to the moment we are awake and stopped. Therefore, that energy we gain when we sleep we could recover it by eating a little more, without having to sleep about 25 years of our life. Sleep must necessarily be a more complex phenomenon, as throughout evolution all animal species have maintained it.

More recent hypotheses indicate that sleep is related to the learning process. Chorizos, for example, when studying chorizo, activate certain areas of the brain. When they are asleep they repeat that same pattern of neuronal activation: the areas used during the day in the brain are activated again, although at night birds do not produce any sound. It is as if they dreamed while singing.

The same phenomenon has been observed with other animal species that are learning any new action: at night it is ‘remembered’. For this reason, many scientists believe it is time to fix the memory, to strengthen what they learned during the day.

Neurobiologists have studied the brains of people who slept and have seen that, although the muscles moved very little, neurons have an enormous activity. But what are they dedicated to?

During the day, when our eyes see something, our ears hear something or our nose cooks something, neurons receive and carry that information to the brain. And precisely, as information must be transmitted from one neuron to another, the relationships between neurons — synapses — are the key to information exchange. Therefore, it seems that when we are sleeping, neural bonds created when we are awake are reviewed and those that are useful and those that are ill-formed are saved. The plasticity of neurons during sleep is the key to the ability to learn new things.

Good moment of sleep

Being neurons the protagonists of sleep, scientists have analyzed in what phase of sleep occurs this adjustment of the synapses. In fact, the REM phase and the NREM phase occur alternately during sleep, a phase with high brain activity and another slow wave phase.

More importance has always been given to the REM phase, on the one hand because it is when we dream and, on the other, because it has always been considered to be fundamental to consolidate what we learned. But the latest research by neurobiologists has shown that the plasticity of neurons occurs in the NREM phase, eliminating the absolute protagonism of dreams.

Without sleep

Scientists have made numerous attempts to verify how the neuronal circuit has influenced the ability to learn new information. Several people have studied the ability to learn new words, comparing those who have slept for hours and those who have slept nothing.

These studies show that people who have not slept need to use more areas
of the brain to learn words, the less asleep the less fields, the more fields. Researchers believe that in cases where this renewal does not occur in neural circuits, the brain must use other fields to learn.

Although neuronal readjustment has a great influence on learning ability, it is still doubted that the plasticity of neurons is the main function of sleep. According to what we have learned throughout the day, if only a part of the brain strengthens the synapses, why is it necessary to sleep every night for other areas of the brain and the body?

Synaptic consolidation may consist of a secondary benefit of the NREM phase, in some way, in the by-product of a process designed for other tasks, as well as the metabolic recovery of neurons. At the moment, sleep remains a completely dark physiological process.

Half sleep, half awake

Dolphins have sought a solution to the need to always be awake at sea: instead of the whole brain, only one hemisphere sleeps. Alternatively, they perform short climbs, first the right hemisphere and then the left. They can continue breathing normally.

Among birds is common hemispheric sleep, essential to spy on predators. It seems very practical. But it is totally unusual among mammals: only dolphins, cetaceans and manatees have this capacity. Its use is surprising, considering that the precursors of mammals, reptiles, also had that capacity, and yet mammals have not remained in evolution. This loss highlights the possible existence of a reason for evolution not to favor the unihemispheric dream, and perhaps only worth it in very extreme ways of life.

Sleepless nights In 1997 a 73-year-old woman suffered a stroke. The occial lobe of the brain ran out of blood and had an unexpected consequence: it stopped dreaming. Since then they have been studying women at the University of Zurich and have seen what is the exact area to dream about in the brain. That is, where is the field that processes emotions and visual memory. Previously, affected brain areas responsible for creating dreams have been studied, but patients, in addition to not being able to sleep, had physical problems. This woman has been the only one who has lost the ability to dream. While we sleep we dream in the REM phase, in which the woman was awakened again and again. Although he did not dream, the recording of the waves was completely normal, just like the REM phase. This discovery has shown that there have been two distinct episodes between sleep and the slow wave phase, that is, they are controlled by different areas of the brain. Sigmund Freud thought that dreams serve to release repressed feelings, while others help classify everyday events or solve problems. However, seeing that 7 years after suffering a stroke, the woman who did not dream has no other type of neurological damage, it seems that dreaming is not at all necessary for mental well-being. Moreover, it may not have any function. Perhaps those who say that dreams are the cinema of the brain are right; somehow, dreams have the function of entertaining the brain while we sleep.