RESETTING THE MELATONIN RHYTHM WITH LIGHT IN HUMANS

The endogenous circadian rhythm of melatonin in humans provides inform ation regarding the resetting response of the human circadian timing s ystem to changes in the Light-dark (LD) cycle. Alterations in the LD c ycle have both acute and chronic effects on the observed melatonin rhy thm. Investigations to date have firmly established that the melatonin rhythm can be reentrained following an inversion of the LD cycle. Exp osure to bright light and darkness given over a series of days can rap idly induce large-magnitude phase shifts of the melatonin rhythm. Even single pulses of bright light can shift the timing of the melatonin r hythm. 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., cone s) is sufficient to reset the endogenous circadian melatonin rhythm. I n addition to phase resetting, exposure to light of critical timing, s trength, and duration can attenuate the amplitude of the endogenous ci rcadian rhythm of melatonin. Measurement of melatonin throughout reset ting trials provides a dynamic view of the resetting response of the h uman circadian pacemaker to light. Future studies of the melatonin rhy thm in humans may further characterize the resetting response of the h uman circadian timing system to light.