On the functional significance of sleep.

Some aspects of the functional significance of sleep are considered. These involve studies on (i) the dynamics of electrical activity in the neo- and archi-paleocortical structures of the brain in the sleep-wakefulness cycle, (ii) the effect of selective deprivation of different sleep phases on the sleep-wakefulness cycle, (iii) effect of stimulating synchronizing and desynchronizing systems on the sleep-wakefulness cycle, and (iv) the dynamics of emotional tension in the sleep-wakefulness cycle. Analysis of our findings and their comparison with those reported in the literature support a hypothesis that every phase in the sleep-wakefulness cycle operates in two directions; it withdraws the factors formed in the previous phase which are potentially aimed at disturbing brain homeostasis; and, at the same time, it forms new antihomeostatic factors, withdrawal of which requires the triggering and work of a subsequent phase. The triggering links of the systems regulating each phase are very sensitive to the factors formed in the preceding phase. Therefore, in normal conditions the anti-homeostatic factors fail to attain a critical level of disturbance of homeostasis of the brain, since the threshold for their activation is far lower than the critical level of homeostatic disturbance. Activation of the triggering links of the paradoxical phase and wakefulness seems to be mediated through the same anti-homeostatic factors formed in the orthodox phase. However, the threshold for activation of the paradoxical phase is lower than that of wakefulness. As a consequence, the orthodox phase readily passes into the paradoxical. The latter, effectively reducing the antihomeostatic factors formed in the orthodox phase, prevents the onset of undue awakening that would have been undesirable because there would have been insufficient time for inactivation of the anti-homeostatic factors formed during wakefulness.