PHASE-SHIFTING THE HUMAN CIRCADIAN CLOCK USING MELATONIN

Melatonin is produced only during nighttime darkness. Its onset during the evening is a useful marker for circadian phase position, when mel atonin levels are sampled frequently and under conditions of dim light or darkness. This marker is termed the dim light melatonin onset (DLM O). Recently, we have described a phase response curve (PRC) to melato nin in humans that is about 12 h out of phase with the PRC to light. E xogenous melatonin administration and bright light exposure can be use d to shift circadian rhythms according to their respective PRCs. These phase-resetting agents can be used alone or together to treat circadi an phase disorders, which include advanced and delayed sleep phase syn drome, shift work maladaptation, jet lag and winter depression. Endoge nous melatonin may function to augment entrainment of circadian rhythm s by the light-dark cycle.