SIMULATIONS OF LIGHT EFFECTS ON THE HUMAN CIRCADIAN PACEMAKER - IMPLICATIONS FOR ASSESSMENT OF INTRINSIC PERIOD

The sensitivity of the human circadian system to light has been the su bject of considerable debate. Using computer simulations of a recent q uantitative model for the effects of light on the human circadian syst em, we investigated these effects of light during different experiment al protocols. The results of the simulations indicate that the nonunif orm distribution over the circadian cycle of exposure to ordinary room light seen in classical free-run studies, in which subjects select th eir exposure to light and darkness, can result in an observed period o f similar to 25 h, even when the intrinsic period of the subject's end ogenous circadian pacemaker is much closer to 24 h. Other simulation r esults suggest that accurate assessment of the true intrinsic period o f the human circadian pacemaker requires low ambient light intensities (similar to 10-15 lx) during scheduled wake episodes, desynchrony of the imposed light-dark cycle from the endogenous circadian oscillator, and a study length of at least 20 days. Although these simulations aw ait further experimental substantiation, they highlight the sensitivit y to light of the human circadian system and the potential confounding influence of light on the assessment of the intrinsic period of the c ircadian pacemaker.