Abstract
Many aspects of human physiology, metabolism, and behavior are dominated by 24-h circadian rhythms including the sleep–wake cycle, alertness and performance patterns, and some hormones. These rhythms are spontaneously generated by an internal circadian clock in the brain and daily light exposure to the eyes synchronizes the clock with the external environment. Most blind people with no perception of light, however, experience continual circadian desynchrony through a failure of light information to reach the circadian pacemaker, resulting in non-24-h sleep–wake disorder. Daily melatonin administration, which provides a replacement synchronizing daily “time cue,” is a promising therapeutic strategy to treat this disorder.