We assessed the activity of the sympathetic nervous system during undisturbed nocturnal sleep and periods of wakefulness directly before and after sleep in healthy young men. Changes induced by periods of rapid eye movement and by morning awakening, both periods reported to demonstrate an enhanced risk for the onset of cardiovascular diseases, were of particular interest. In 13 healthy men (age, 18 to 35 years), blood for determination of epinephrine and norepinephrine was drawn every 7 minutes between 9:30 PM and 8:30 AM, with the subjects resting in a strictly horizontal position. Lights were switched off at 11 PM until awakening at 7 AM. At 8:30 AM, subjects stood up and a final blood sample was drawn. Sleep was monitored somnopolygraphically, and heart rate and blood pressure were continuously measured. Average epinephrine but not norepinephrine concentrations were significantly lower during nocturnal sleep than during wakefulness before and after sleep. In parallel, heart rate and blood pressure declined significantly during sleep. During rapid eye movement sleep, both epinephrine and norepinephrine concentrations were significantly lower than during sleep stages 1 and 2 and slow-wave sleep. Whereas epinephrine concentrations gradually began to increase after morning awakening, norepinephrine levels were not significantly enhanced. However, standing up at the end of the experiment sharply increased norepinephrine concentrations by 180%, whereas epinephrine levels were less enhanced (46%) by the change of body position. This study suggests that the decrease in the activity of the sympathoadrenal branch of the sympathetic nervous system is probably due to an entrainment to the sleep-wake cycle, whereas the low activity of the noradrenergic branches depends mainly on horizontal body position during nocturnal sleep. The activities of the sympathoadrenal and noradrenergic branches of the sympathetic nervous system seem to be downregulated during rapid eye movement sleep. Awakening itself selectively enhances epinephrine levels. Subsequent orthostasis activates both the sympathoadrenal and, most prominently, the noradrenergic branches of the sympathetic nervous system. (Hypertension. 1997;30[part 1]:71-76.)