The human circadian pacemaker can see by the dawn's early light

The authors' previous experiments have shown that dawn simulation at low li ght intensities can phase advance the circadian rhythm of melatonin in huma ns. The aim of this study was to compare the effect of repeated dawn signal s on the phase position of circadian rhythms in healthy participants kept u nder controlled light conditions. Nine men participated in two 9-day labora tory sessions under an LD cycle 17.5:6.5 h, < 30:0 lux, receiving 6 consecu tive daily dawn (average illuminance 155 lux) or control light (0.1 lux) si gnals from 0600 to 0730 h (crossover, random-order design). Two modified co nstant routine protocols before and after the light stimuli measured saliva ry melatonin (dim light melatonin onset DLMOn and offset DLMOff) and rectal temperature rhythms (midrange crossing time [MRCT]). Compared with initial values, participants significantly phase delayed after 6 days under contro l light conditions (at least -42 min DLMOn, -54 min DLMOff, -41 min MRCT) i n spite of constant bedtimes. This delay was not observed with dawn signals (+10 min DLMOn, +2 min DLMOff, 0 min MRCT). Given that the endogenous circ adian period of the human circadian pacemaker is slightly longer than 24 h, the findings suggest that a naturalistic dawn signal is sufficient to fore stall this natural delay drift. Zeitgeber transduction and circadian system response are hypothesized to be tuned to the time-rate-of-change of natura listic twilight signals.