DOSE-RESPONSE RELATIONSHIPS FOR RESETTING OF HUMAN CIRCADIAN CLOCK BYLIGHT

SINCE the first report in unicells(1), studies across diverse species have demonstrated that light is a powerful synchronizer which resets, in an intensity-dependent manner, endogenous circadian pacemakers(1-5) . Although it is recognized that bright light (similar to 7,000 to 13, 000 lux) is an effective circadian synchronizer in humans(6-10), it is widely believed that the human circadian pacemaker is insensitive to ordinary indoor illumination (similar to 50-300 lux)(11). It has been proposed that the relationship between the resetting effect of light a nd its intensity follows a compressive nonlinear function(12), such th at exposure to lower illuminances still exerts a robust effect(13), We therefore undertook a series of experiments which support this hypoth esis and report here that light of even relatively low intensity (simi lar to 180 lux) significantly phase-shifts the human circadian pacemak er. Our results clearly demonstrate that humans are much more sensitiv e to light than initially suspected and support the conclusion that th ey are not qualitatively different from other mammals in their mechani sm of circadian entrainment(14).