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