A Phase Response Curve to Single Bright Light Pulses in Human Subjects

A significant peak-to-trough amplitude of 5.02 hours was observed in the phase response curve (PRC) of melatonin rhythm with varying light exposure timing.

• Light exposure's effectiveness in resetting the circadian pacemaker varies based on the circadian phase when exposure occurs.
• Phase delays were noted when light was administered before the critical phase at the body temperature minimum.
• Phase advances were observed when light exposure occurred after this critical phase.
• No phase shift was recorded at the critical phase itself.
• Larger phase shifts were detected from melatonin onset measurements compared to offsets.
• No prolonged period of photic insensitivity, or 'dead zone', was found during the subjective day.

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