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To evaluate the relative contribution of sleep and the endogenous circadian rhythmicity in producing the 24-h variations in the plasma renin activity.Ten normal young men were studied, under basal conditions with normal nocturnal sleep from 2300–0700 h and once after a night of total sleep deprivation followed by 8 h daytime sleep from 0700 to 1500 h. Plasma renin activity was measured every 10 min for 24 h and the profiles were analysed using the pulse detection program ULTRA.During the 8 h night-time sleep a significant increase in the mean plasma renin activity levels occurred compared with the subsequent 8-h waking periods. After the shift in the sleep period, a sleep-associated increase was clearly apparent during the daytime hours. The number and the amplitude of the oscillations, linked to the non-rapid eye movement-rapid eye movement sleep cycles, increased during sleep (at whatever time it occurred), and were dependent on the regularity and the length of the sleep cycles. In awake subjects the plasma renin activity generally fluctuated in a more damped and irregular manner, but occasionally the plasma renin activity oscillated at a regular periodicity with two dominant peaks centred around 100 and 50 min.These results demonstrate that the 24-h plasma renin activity variations are not circadian in nature but are related to sleep processes, which create the nycthemeral rhythm by increasing both the frequency and the amplitude of the oscillations.