Entrainment of Human Orcadian Rhythms by Light-Dark Cycles: A Reassessment

— Man is the only eukaryotic organism in which it has been reported that the circadian system cannot be entrained to a 24-h period by a simple light-dark (LD) cycle. In this paper, we reexamine the evidence for that claim and demonstrate that there were some fundamental flaws in the experimental design of the previous studies on which this conclusion was based. We report new studies in which we tested the efficacy of LD cycles in entraining the circadian rhythms of human subjects living in isolation from environmental time cues. We found that the cyclic alternation of light and dark, when applied to human subjects in a comparable way to experiments in other species, was an effective entraining agent. Our results and a critical review of the literature indicate that a LD cycle alone can be an effective environmental synchronizer of the human circadian timing system. Other factors, such as the knowledge of time of day, social contacts, the feeding schedule, and the imposed rest-activity schedule may contribute to stable entrainment, although their relative strengths as synchronizers have yet to be determined.