Release of melatonin into the circulation by the pineal occurs almost exclusively during the nighttime hours. It has been proposed that this daily rhythm, like that of body temperature, reflects the output of a central circadian pacemaker in humans. In order to investigate the relationship of the circadian rhythms of body temperature and melatonin in humans and compare their resetting responses to light, we characterized the endogenous 24-h profiles of these rhythms in eight young male adults during constant routines before and after exposure to a stimulus consisting of bright light, room light, and darkness/sleep. We found that the time of the fitted maximum of the endogenous melatonin rhythm consistently preceded the fitted temperature minimum by a mean +/- SE of 1.8 +/- 0.2 h. Bright-light exposure induced substantial and equivalent phase shifts of the melatonin and temperature rhythms (mean +/- SE difference in the phase-shifting response, 0.03 +/- 0.32 h), and the body temperature and melatonin rhythms thus maintained their usual phase relationship even after light-induced circadian phase inversion. These results are consistent with the hypothesis that the endogenous circadian components of both the plasma melatonin and body temperature rhythms are generated by a single central circadian pacemaker in humans. Furthermore, using the time of the fitted temperature minimum as a reference standard, we found that the fitted maximum of the endogenous 24-h melatonin profile was a more reliable phase marker than the onset of the nocturnal rise of melatonin (F = 4.48; P less than 0.01).