The start of the quiescent period of cortisol remains phase locked to the melatonin onset despite circadian phase alterations in humans working the night schedule.

Using a 10-min blood sampling procedure, we established 24-h plasma melatonin and cortisol rhythms in 11 night workers and determined whether the extent in the shift of the melatonin onset, highly variable among night workers, was reflected in the shift of the markers of the cortisol rhythm, i.e. the quiescent period of secretion and the acrophase. In all day-active subjects, the melatonin onset occurred during low cortisol secretion, with a time lag between the start of the quiescent period and the melatonin onset of 1 h 28+or-27 min. In night workers, whatever the shift of the melatonin surge, the start of the quiescent period of cortisol secretion remained phase locked to the melatonin onset with a similar time lag (1 h 25+or-27 min). There was a significant correlation between the timing of the melatonin onset and the timing of the start of the quiescent period (r=0.88; P=0.0072). No preserved time lag was found between the melatonin onset and the other cortisol phase markers, either with the end of the quiescent period or with the acrophase. These results settle the start of the quiescent period of cortisol and the melatonin onset as two coordinate markers, and suggest that each of them are reliable to assess circadian phase in humans.