Phase advance after one or three simulated dawns in humans

A specially designed apparatus that can simulate the waveform of the dawn o r dusk signal at any latitude and any day of the year has been shown to pha se shift the circadian pacemaker in rodents and primates at a fraction of t he illuminance previously used. Until recently, it was considered that rath er high illuminances or rather long exposure episodes to room light were ne cessary to phase shift human circadian rhythms. This experiment shows that, under controlled conditions of a modified constant routine protocol, a sin gle dawn signal is sufficient to phase advance the timing of the onset of s ecretion of the pineal hormone melatonin. The significant phase advance of salivary melatonin of 20 minutes, which is enhanced to 34 minutes after thr ee consecutive dawn signals, is small, but appears to be of sufficient magn itude to entrain the human circadian pacemaker, which has an endogenous per iod of about 24.2h.