Schematic representation of the phase response curves to light and melatonin. Circadian time point 0 reflects the temperature nadir. Light exposure prior to the temperature nadir results in a delay of circadian rhythms, whereas light exposure after the temperature nadir causes phase advances. Note that there is a dead zone in the middle of the day where bright light exposure has no effect on the timing of circadian rhythms. In contrast, melatonin administered in the beginning of the night advances the circadian rhythm, while melatonin in the morning delays the circadian rhythm.
Adapted from:
Lewy AJ, Bauer VK, Ahmed S, et al. The human phase response curve (PRC) to melatonin is about 12 hours out of phase with the PRC to light. Chronobiol Int 1998; 15:71.
Khalsa SB, Jewett ME, Cajochen C, Czeisler CA. A phase response curve to single bright light pulses in human subjects. J Physiol 2003; 549(Pt 3):945.
Reproduced from: Zee PC, Manthena P. The brain's master circadian clock: Implications and opportunities for therapy of sleep disorders. Sleep Med Rev 2007; 11:59. Illustration used with the permission of Elsevier Inc. All rights reserved.
