Taking melatonin at night changes how the body’s internal clock responds to bright light

Bright light exposure advanced daily rhythms by about 90-120 minutes in body temperature, cortisol, and melatonin.

• Bright light stimuli significantly advanced core body temperature, cortisol, and melatonin rhythms.
• Administration of melatonin during light exposure reduced the phase advances of cortisol and body temperature rhythms.
• The peak and offset of melatonin rhythms were also antagonized by contemporaneous melatonin administration.
• Conversely, the onset of melatonin was enhanced with melatonin administration during bright light exposure.
• Contemporaneous melatonin administration modifies the effects of light on circadian phase shifts.

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