Light Exposure Induces Equivalent Phase Shifts of the Endogenous Circadian Rhythms of Circulating Plasma Melatonin and Core Body Temperature in Men*

The fitted maximum of the endogenous melatonin rhythm occurs 1.8 hours before the fitted minimum of body temperature.

• Bright-light exposure resulted in comparable phase shifts in both melatonin and body temperature rhythms.
• The mean difference in the phase-shifting response to bright light was 0.03 hours.
• The phase relationship between melatonin and body temperature rhythms remained consistent even following light-induced phase shifts.
• The fitted maximum of the 24-hour melatonin profile was found to be a more reliable phase marker than the onset of melatonin's nocturnal rise.

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