NONPHOTIC ENTRAINMENT OF THE HUMAN CIRCADIAN PACEMAKER

In organisms as diverse as single-celled algae and humans, light is th e primary stimulus mediating entrainment of the circadian biological d ock. Reports that some totally blind individuals appear entrained to t he 24-h day have suggested that nonphotic stimuli may also be effectiv e circadian synchronizers in humans, although the nonphotic stimuli ar e probably comparatively weak synchronizers, because the circadian rhy thms of many totally blind individuals ''free run'' even when they mai ntain a 24-h activity-rest schedule. To investigate entrainment by non photic synchronizers, we studied the endogenous circadian melatonin an d core body temperature rhythms of 15 totally blind subjects who lacke d conscious light perception and exhibited no suppression of plasma me latonin in response to ocular bright-light exposure. Nine of these fif teen blind individuals were able to maintain synchronization to the 24 -h day, albeit often at an atypical phase angle of entrainment. Nonpho tic stimuli also synchronized the endogenous circadian rhythms of a to tally blind individual to a non-24-h schedule while living in constant near darkness. We conclude that nonphotic stimuli can entrain the hum an circadian pacemaker in some individuals lacking ocular circadian ph otoreception.