<qa>  What is the effect on the human circadian system of different durations of dim-light sensThe 13-day inpatient studies the investigators propose will examine the effect on the human circadian system of different durations of dim-light sensitization prior to a standardized light treatment. These results will be compared within subjects in a randomized cross-over design study in which each subject will receive a control treatment and a light treatment with prior dim-light sensitization. The investigators will also include circadian phase disorder patients to test these mechanisms in the target patient population. Our findings will provide an important step in understanding how new knowledge about the circadian photoreceptive system can be used to refine and provide better treatment options for circadian rhythm disorders. 