Adaptation of bodily functions to sleep

Behavioural states can be identied according to the value of different physiological variables, driven by the integrated activity of the somatomotor, autonomic and endocrine systems. This integration occurs mainly at the hypothalamic level. Therefore, the physiological denition and understanding of wake–sleep (WS) states is possible only through 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 denition of the WS states, based substantially on the level of brain cortical and somatomotor activity. According to this, non-rapid eye movement (NREM) sleep can be dened 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 reexes. During NREM sleep, physiological regulation is clearly operant for the maintenance of body homeostasis. Conversely, during rapid eye movement (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 dened 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.