Sleep consists of two phases that periodically alternate: the rapid eye movement (REM) phase and the non-REM phase. The non-REM stage is characterized by wide synchronous waves in the electroencephalogram, by a low heart rate and by a decrease in arterial blood pressure and peripheral resistances. This hemodynamic setting is the consequence of the autonomic balance characterized by high vagal activity and low sympathetic activity. Such an autonomic condition is adequately described by the spectral analysis of heart rate variability documenting a prevalence in the high frequency band (the respiratory vagal band). The REM stage of sleep is characterized by asynchronous waves in the electroencephalogram and it is associated with a further increase in the vagal dominance of the autonomic balance resulting in a lower heart rate and decreased peripheral resistances. The REM phase of sleep is, however, also characterized by hemodynamic instability due to sudden bursts of sympathetic activity, associated with the rapid eye movements. These sympathetic bursts cause sudden changes in heart rate and peripheral resistance and may influence cardiac electrical stability both at the atrial and ventricular levels. Additionally, REM sleep may enhance the risk of anginal attacks in coronary artery disease patients. Analysis of the autonomic balance during the different phases of sleep may also help in the identification of autonomic derangements typically associated with myocardial infarction.