THE HUMAN PHASE RESPONSE CURVE (PRC) TO MELATONIN IS ABOUT 12 HOURS OUT-OF-PHASE WITH THE PRC TO LIGHT

Melatonin's timekeeping function is undoubtedly related to the fact th at it is primarily produced during nighttime darkness; that is, melato nin and light occur at opposite times. The human phase response curve (PRC) to melatonin appears to be about 12h out of phase with the PRC t o light. These striking complementarities, together with light's acute suppressant effect on melatonin production, suggest that a function f or endogenous melatonin is to augment entrainment of the circadian pac emaker by the light-dark cycle. The melatonin PRC also indicates corre ct administration times for using exogenous melatonin to treat circadi an phase disorders.