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

A significant correlation (r=0.88; P=0.0072) exists between the timing of melatonin onset and the start of the quiescent period of cortisol secretion.

• In day-active individuals, melatonin onset occurs during low cortisol secretion, with an average time lag of 1 hour and 28 minutes.
• For night workers, the start of cortisol's quiescent period remains phase-locked to melatonin onset, with a similar lag of 1 hour and 25 minutes.
• No preserved time lag is observed between melatonin onset and other cortisol phase markers, such as the end of the quiescent period or the peak secretion time.
• The results indicate that the start of cortisol's quiescent period and melatonin onset are reliable markers for assessing circadian phase in humans.

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