The pulsatile patterns of cortisol release have been referred to as ‘episodic’ rather than ‘rhythmic’. We studied plasma cortisol patterns in 31 healthy subjects during night sleep and observed a uniform ultradian rhythm. Using spectral analysis a predominant mean periodicity of 151.2 ± 8.3 min (x ± SEM) was calculated for plasma cortisol oscillations, and of 118.3 ± 4.3 min for REM sleep occurrence. The rhythmic pattern of plasma cortisol was obscured when data were averaged across subjects, due to the individual onset latencies of the initial cortisol rise. Therefore, the data were analyzed time-locked to a number of somnopolygraphic sleep events: by selecting the end of the second sleep cycle as point of reference, the ultradian rhythm was preserved. These results indicate that both sleep processes and nocturnal plasma cortisol concentrations are subject to ultradian rhythms. Both rhythms are coupled, but have different periodicities.