The endogenous circadian rhythm of melatonin in humans provides information regarding the resetting response of the human circadian timing system to changes in the light-dark (LD) cycle. Alterations in the LD cycle have both acute and chronic effects on the observed melatonin rhythm. Investigations to date have firmly established that the melatonin rhythm can be reentrained following an inversion of the LD cycle. Exposure to bright light and darkness given over a series of days can rapidly induce large-magnitude phase shifts of the melatonin rhythm. Even single pulses of bright light can shift the timing of the melatonin rhythm. Recent data have demonstrated that lower light intensities than originally believed are capable of resetting the melatonin rhythm and that stimulation of photopically sensitive photoreceptors (i.e., cones) is sufficient to reset the endogenous circadian melatonin rhythm. In addition to phase resetting, exposure to light of critical timing, strength, and duration can attenuate the amplitude of the endogenous circadian rhythm of melatonin. Measurement of melatonin throughout resetting trials provides a dynamic view of the resetting response of the human circadian pacemaker to light. Future studies of the melatonin rhythm in humans may further characterize the resetting response of the human circadian timing system to light.
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