Adaptation of bodily functions to sleep

Behavioral states can be identified according to the value of different physiological variables, driven by the integrated activity of the somatomotor, autonomic and endocrine systems. This integration mostly occurs at the hypothalamic level. Therefore, the physiological definition and understanding of wake-sleep (WS) states is possible only through the assessment, during the different states, of the respiratory, cardiovascular, and metabolic parameters and the responses elicited by internal or external stimuli challenging the different physiological regulatory mechanisms. This goes far beyond the standard definition of the WS states, substantially based on the level of brain cortical and somatomotor activity. According to this, non-REM sleep can be defined as a state of minimal energy expenditure and motor activity, during which cardiovascular, respiratory and thermoregulatory variables are driven by the autonomic nervous system at a lower level compared to wakefulness and are kept stable by the autonomic reflexes. During non-REM sleep, physiological regulation is clearly operant for the maintenance of body homeostasis. On the contrary, during REM sleep posture control is lost, autonomic activity is highly unstable, centrally driven surges in heart rate and blood pressure occur, breathing becomes irregular, and thermoregulation is suspended or depressed. Such an operative modality of physiological regulation has been defined as “poikilostatic” and attributed to a derangement of the integrative function of the hypothalamus during REM sleep. However, recent data showing that osmoregulation is maintained during REM sleep suggest that such a derangement is not extended to the whole hypothalamus.