HUMAN CIRCADIAN PACEMAKER IS SENSITIVE TO LIGHT THROUGHOUT SUBJECTIVEDAY WITHOUT EVIDENCE OF TRANSIENTS

Fifty-six resetting trials were conducted across the subjective day in 43 young men using a three-cycle bright-light (similar to 10,000 lx) stimulus against a background of very dim light (10-15 lx). The phase- response curve (PRC) to these trials was assessed for the presence of a ''dead zone'' of photic insensitivity and was compared with another three-cycle PRC that had used a background of similar to 150 lx. To as sess possible transients after the light stimulus, the trials were div ided into 43 steady-state trials, which occurred after several baselin e days, and 13 consecutive trials, which occurred immediately after a previous resetting trial. We found that I) bright light induces phase shifts throughout subjective day with no apparent dead zone; 2) there is no evidence of transients in constant routine assessments of the fi tted temperature minimum 1-2 days after completion of the resetting st imulus; and 3) the timing of background room light modulates the reset ting response to bright light. These data indicate that the human circ adian pacemaker is sensitive to light at virtually all circadian phase s, implying that the entire 24-h pattern of light exposure contributes to entrainment.