Editing Phase response curve (section)

===Melatonin===

The phase response curve for melatonin is roughly twelve hours out of phase with the phase response curve for light.<ref>{{cite journal | vauthors = Lewy AJ, Ahmed S, Jackson JM, Sack RL | title = Melatonin shifts human circadian rhythms according to a phase-response curve | journal = Chronobiology International | volume = 9 | issue = 5 | pages = 380–92 | date = October 1992 | pmid = 1394610 | doi = 10.3109/07420529209064550 }}</ref> At spontaneous wake-up time, [[exogenous]] (externally administered) melatonin has a slight phase-delaying effect. The amount of phase-delay increases until about eight hours after wake-up time, when the effect swings abruptly from strong phase delay to strong phase advance. The phase-advance effect diminishes as the day goes on until it reaches zero about bedtime. From usual bedtime until wake-up time, exogenous melatonin has no effect on circadian phase.<ref>{{cite journal | vauthors = Burgess HJ, Revell VL, Eastman CI | title = A three pulse phase response curve to three milligrams of melatonin in humans | journal = The Journal of Physiology | volume = 586 | issue = 2 | pages = 639–47 | date = January 2008 | pmid = 18006583 | pmc = 2375577 | doi = 10.1113/jphysiol.2007.143180 }}</ref><ref>{{cite journal | vauthors = Lewy A | title = Clinical implications of the melatonin phase response curve | journal = The Journal of Clinical Endocrinology and Metabolism | volume = 95 | issue = 7 | pages = 3158–60 | date = July 2010 | pmid = 20610608 | pmc = 2928905 | doi = 10.1210/jc.2010-1031 }}</ref>

The human body produces its own ([[endogenous]]) melatonin starting about two hours before bedtime, provided the lighting is dim. This is known as ''dim-light melatonin onset'', DLMO.<ref>{{cite journal | vauthors = Sletten TL, Vincenzi S, Redman JR, Lockley SW, Rajaratnam SM | title = Timing of sleep and its relationship with the endogenous melatonin rhythm | journal = Frontiers in Neurology | volume = 1 | pages = 137 | date = 2010 | pmid = 21188265 | pmc = 3008942 | doi = 10.3389/fneur.2010.00137 | doi-access = free }}</ref> This stimulates the phase-advance portion of the PRC and helps keep the body on a regular sleep-wake schedule. It also helps prepare the body for sleep.

Administration of melatonin at any time may have a mild [[hypnotic]] (sleep-inducing) effect. The expected effect on sleep phase timing, if any, is predicted by the PRC.