ABSTRACT
NORMAL SLEEP Normal sleep is divided into REM (associated with rapid eye movements) and non-REM (NREM) stages: NREM sleep is further divided into stages I, II, III and IV, representing progressively deeper stages of sleep, with gradual reductions in sympathetic activity and consequently in heart rate, stroke volume, cardiac output, systemic vascular resistance and blood pressure. Simultaneously, ventilation and metabolic rate also decline. During stage IV of sleep sympathetic activity, heart rate, blood pressure and ventilation are usually at their nadir. Conversely, parasympathetic or vagal activity increases in NREM sleep, particularly in stage IV.
