In the evolutionary plan the sleep of warm-blooded animals, which has rapid (paradoxical, REM) and slow phases, became its kind a compromise between the need for leisure from one side and the requirement of safety, the need for adaptation to cold during the sleep - with another. It seems that the slow phase - major, most ancient portion of the sleep. However, rapid sleep in connection with teplokrovnost'yu could arise from the mechanism of the similar to warming-up on leaving from the state of the hibernation (hibernation - this retarding of all functions with a decrease in the temperature of body). Possibly, upon transfer to teplokrovnosti there were forms of already warm-blooded animals, whose daily sleep was still similar to the torpor (some very small warm-blooded animals have a similar torpor it is observed now). Such sleep- torpor in some stage of evolutionary development could have not only daily rhythm, but also repeated cycles (torpor - warming-up) during 24 hrs or only at night. I.e. as in the contemporary structure of sleep, which proved to be very valuable acquisition and it was fastened in the predominant number it was specific warm-blooded animals.
The rapid phase of sleep guarantees the instantaneous return of consciousness, capability for orientation with the awakening during or at the end of this phase. The paralyzed state during this active phase makes possible for animals to remain concealed before the predators. All this increases safety, and, possibly, even the rapid motions of eyes in that sleeping, seen for those surrounding (signal for that approaching, which sleeping any minute now will awake), is fulfilled the same function.
Another change of the cycles of slow and rapid phases makes it possible not to allow supercooling, which can begin during the slow phase of sleep. For example, in elephants (by which because of its mass it is difficult to be supercooled) this cycle reaches 2 hours. For the comparison: in small animals it can last not more than 10 minutes. Moreover not less than fourth cycle occupies the period of "warming-up" - i.e., rapid phase. In man the cycle of sleep occupies intermediate position - about 90 minutes.
For leisure and restoring the mental functions more important, apparently, is slow sleep this they confirm experiments with the deprivation of sleep. During this phase is observed the deepest braking CNS (Central Nervous System), they are neutralized (they are displaced) the experience of the past day, which contributes to the restoration of the mental equilibrium (it is sufficiently and several minutes of drowsy, surface sleep). However, the stronger, more urgent experiences, which did not pass the "sieve" of slow sleep, "are caught" by the subsequent rapid phase of sleep and they become the theme of dreams. But dreams, in turn, contribute to the displacement of these experiences ("processing" of information) by realizing the "symbols" or increases the probability of realizing these "symbols" during the wakefulness (possibly, when sleeps are not memorized). All these processes remove stress, and, at the same time, they contribute to retention in the memory it is indicative likely danger. Frequently in the dreams one subject can contain many different agitating themes, connected with many associations, into itself. This concentration of events in the uncommon combinations - thickening, dramatization (terms from the work of Z. Freud) - strengthen emotional effect. Specifically, such sleeps most frequently and are memorized, i.e., they are realized. The greater the associations, the more important the information and, therefore, the more it noteworthy.
Consequently, dream can have preventive function. For example, the nightmares of drops strengthen and is supported the fear of height at the necessary level. In this case the reaction of pain, previously appeared with the drops, prove to be that extruded from the consciousness (it it is absent also in the subject of dream), but is symbolically represented in the dream in the form of fear with the drop. The adaptive nature of similar displacements does not cause doubt. If animals or man are constant, in any situation fear possible damages and painful reactions, this will forge motions and it will lower search activity. Nightmares of the type of "drops", and also displaced means in the softer form (symbols) are intended precisely in order to place "markers" on the actually dangerous situations in the form of the fear of height, falling objects, or in other more complex vital situations.
Let us examine how the conditional reaction of fear in the situations with the drops can be formed. In the real life nothing is repeated in the same situation several times. Yes even situation itself changes. From the generation the children do not fear height, but when they begin to walk - much fall. For example, one time child fell and tested the pain, when rapidly he broke into a run after the cat, in other case it fell from stul'chika, and on the street - from the children's swing. In each of these situations constantly was present only one prearranged signal - loss of support. To it is manufactured the conditioned reflex or the reaction of fear. (See: Reflexes, emotions, motives.)
What adaptive value can have a fear of the loss of support, indeed child does not realize that he when - or did lose support and will not foresee this in the dangerous situations? In fact, in view of the transience, the very short duration of drop itself, is not realized, apparently, and very fear of the loss of support. Answer to this question should be searched for, most likely, in the dreams. Those associations, which appear with each drop: the loss of support and cat, loss of support and stul'chik, loss of support and swing - sooner or later derive on the theme of the loss of support in the dream, and it means - on the theme of drops. Hence the nightmares of drops. Sleep "strengthens" hardly the glowing sensation of danger to such scales, to this fear of drops, that further conditionality, i.e., displacement to the prearranged signal of height, and fear of height, are no longer caused any questions. Like the great adaptive value of the fear of height for averting the drops. Is possible, the "amplifying property of sleep", it is the reason for the difficult extinction of social, etc. it is insurance (phobias).
Running in forward, let us note the rigid tying of the basic (in the childhood) subjects of dreams with the manifestations of gravity. The same fear of drops, falling objects, fear of close spaces, connected with the danger of caving shelters - all testify about this (see: Children's fears, dreams, nightmares.).
It is known that living in the water sea mammals do not have the rapid (REM) phase of sleep (Mukhametov L., 1985), during which are generated the warning dreams. Since the manifestations of gravity in the water are weakened (dangers, connected with it), then it is possible to say that the dreams and rapid phase initially arose (together with other reasons) as the mechanism of warning the connected with the gravity dangerous situations, the probability of occurence of which grew with the advent of teplokrovnosti and, therefore, by the increased activity.
If we investigate dreams not simultaneously, but in the temporary development, then the certain dynamics is observed, for example, in the theme of drops: it is first, predominantly at the childhood, taken the nightmares of "simple" drops; in proportion to vzrosleniya in the stories about the seen sleeps increasingly more is done the accent at the cases of the avoidance of drops, for example, with the walking on the edge of breaks, the roofs of high houses, etc., experiencing in this case the terrible fear, which, as, and prevents drop; further the sleep with the wonderful flights above the surrounding locality or realistic sleep on the same theme, painted with positive emotions, can dream in proportion to the eradication of fear. It is possible to say that this is one and the same sleep, which was extended for many years or, that this is one and the same mental process, which under the action of the surrounding situation undergoes the phase changes (see: Displacement, inversion, sublimation...). The attempt to analyze this phenomenon became the basis of present article.
Dreams imitate real events, i.e., is used that language, which is accessible to us. Function of the brain - perception of the real events of proceeding one after another, reaction on the situation. When appeared the need for (in the phylogenetic sense) tracking the difficultly forecasted, dangerous situations (for example, connected with the gravity), which in reality appear sufficiently rarely so that the necessary experience would be made, then appeared the mechanism of dream. Thus, the function of dream differs somewhat from the function of consciousness, and means the same: the ability to receive to the turn of events, let and generated (example to expediency in living nature: the use of the existing resources, mechanisms for the accomplishment of new or additional objectives).Based on materials of Vladimir Ivanov's article