Resetting the Melatonin Rhythm with Light in Humans

Alterations in the light-dark cycle can induce large-magnitude phase shifts in the melatonin rhythm.

• The melatonin rhythm can be reentrained following an inversion of the light-dark cycle.
• Exposure to bright light and darkness over several days can rapidly shift the timing of the melatonin rhythm.
• Single pulses of bright light may also affect the timing of the melatonin rhythm.
• Lower light intensities than previously thought can reset the melatonin rhythm.
• Stimulation of photopically sensitive photoreceptors (cones) is sufficient to reset the endogenous circadian melatonin rhythm.
• Light exposure can not only phase reset but also reduce the amplitude of the melatonin rhythm.

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