Muscle sympathetic activity (MSA) was recorded in the peroneal nerve during sleep in 14 sleep-deprived healthy subjects. Continuous noninvasive recordings of finger blood pressure were obtained in 7 subjects. In light sleep (stage 2 sleep) the number of sympathetic bursts/min decreased to 90 +/- 8% (mean +/- SEM) and total MSA (= burst/min x mean burst area) to 89 +/- 5% of the awake value (P less than 0.05, n = 14). In deep sleep (stage 3-4) total MSA decreased further, to 71 +/- 8% of the awake value (n = 5). There was no close correlation between variations of depth of sleep and variations of sympathetic activity but during continuously deepening sleep MSA decreased progressively with time. In stage 2 sleep, high amplitude K complexes were accompanied by short-lasting increases of sympathetic activity. Since these increases of MSA were not preceded by decreases of diastolic blood pressure, which is known to evoke increased sympathetic nerve traffic in muscle nerves, we suggest that K complex related increases of MSA are signs of arousal which elicit both cortical EEG phenomena and activation of cerebral sympathetic centres. During desynchronized (REM) sleep, total MSA increased to 124 +/- 12% of the value in awake state (n = 5). The increases occurred mainly in short irregular periods, often related to rapid eye movements and there was an inverse relationship between the duration of the desynchronized sleep and the increase of total MSA. Our findings are similar to the data obtained in animal experiments and may partly explain changes of blood pressure during synchronized and desynchronized sleep reported previously in man.