Why do we fall asleep?

A hundred years ago, scientists thought that the dream is a kind of poisoning. During wakefulness, as was considered, in the body accumulate some poisons the blood brings them to the brain that makes him fall asleep. During sleep poisons are destroyed, removed from the blood, and in the morning we again fresh and active. Were the facts confirming this view, but there were facts that in the end it denied (in particular, observations of fused twins). So, in Moscow before the war was watching a rare case of double ugliness. Born two girls named Ira and Galya, which coalesced in the lower part of the body. They had two heads, with four handles, two hearts and legs and lower body were common. Lived these twins 1 year and 22 days, died from pneumonia. Observations have shown that when one head is sleeping, the other can laugh, to stay awake. It is clear that if sleep onset depended on poisons, being carried by blood and blood from both preorganized was shared, mixed), both heads would have to sleep always synchronous. Scientists of former times could not understand causes of sleep, since I was his nature, did not know how sleep is associated with other properties of the brain. Truly scientific theory of the dream began to take shape only by the works of I. P. Pavlov. For understanding the nature of sleep and dreams, we need to remember and to consider two points.
First, thanks to the conditioned reflex relations between the traces of various impressions brain captures, and most importantly, can, if necessary, re-use any came to it information. From the cradle, when there are first Association (child connects the feeling of the mother with the external appearance, tone of voice, the word "mother"), and then throughout the life of the new that comes in the brain, leave there mark and has intercourse with that old experience available. One impression and put it on another, and eventually the cortex becomes a person like a kind of brain film with hundreds of billions of "frames" - traces and associative relationships between them. That is sealed and can be extracted in the light of all that we saw, heard, read, thought, experienced throughout life, whole life experience.
Secondly, on the basis of the laws of the processes of excitation and braking I. P. Pavlov concluded that sleep is the braking process, covering the main areas of the cortex and descended down to the middle brain. Pavlov decisively rejected the idea about the existence of special courts in the brain responsible States of wakefulness and sleep in those years there was considerable discussion in the literature.
Pavlovskaya theory sleep was an important milestone in the development of neurophysiology, but today has significantly revised. Revealed important function of the brain structures that are grouped under the name of the reticular formation or reticular formation. These structures affect the degree of activity of the cortex of the brain, and can be distinguished centers, the initiation of which is connected with the waking state, and centers that have a right to be called centers of sleep. Was not confirmed and the idea of the dream state as generalized inhibition of the cerebral cortex. Special studies have shown that covered the sleeping brain, the share of the excited nervous centers even more than in the waking state. How did you manage to get such information?
Invaluable assistance in this matter has provided scientists with the currents of the brain. In the brain of the animal implanted to 100 microelectrodes who reported on the state of the electrical activity of the corresponding brain structures. These are "spies" and brought the news that the dream is not connected with the deceleration of the most cortical centers. Pavlovskoe the provision that the dream is not a passive state of the nervous tissue, and active restoration work of the brain, brilliantly confirmed, but not in the sense, as I thought scientist. He spoke about the active nature of the braking process, covering the brain. It has turned out that and braking-when immersed in the dream is in principle not more than in the waking state, i.e. the activity of the sleeping brain is the activity genuinely excited nervous tissue. As all of the above to understand?
In the study of healthy people the currents record from the surface of the head where hair parted, impose plate electrodes. Each electrode collects information from billions of nerve cells. Monitoring has shown that the more active the waking state, besporyadochnye game teeth on the electroencephalogram (so-called record of currents). Prevailing are the levels of frequencies of these waves from 10 to 40 per second. On the contrary, when a person is deeply asleep, marked correct slow wave frequency 2-4 Hz - the so-called Delta waves. A similar picture electroencephalogram means that billions of brain cells work now as well organized, they are synchronized, while in activity in the waking state they operate asynchronously.
Today neuroscientists believe that in mind, there is two systems. Desynchronizing system covers the brain mechanisms that operate within the period of wakefulness and liaise with the external environment; in this system enabled the centers of wakefulness. Another, the clock, the system contains the brain mechanisms during sleep and related to the relevant centers that speed up its offensive. Change of excitation and inhibition is the real fact. But is it that when you Wake excited desynchronizing and slow sync system, and the development of a dream, on the contrary, excited synchronization mechanisms and slow down desynchronizing. In our brain live like two brain - day and night, regularly changing each other on the watch life.