Circadian rhythm sleep disorders occur when individuals attempt to sleep at the wrong circadian time. The misalignment between the internal circadian timing system and the external environment is typically due to either an alteration in the functioning of the circadian timing system (e.g., delayed or advanced sleep phase disorder) or to changes in the external environment (e.g., jet lag). However, the clinical presentation of most of the circadian rhythm sleep disorders is influenced by a combination of physiologic, behavioral, and environmental factors. These disorders lead to complaints of insomnia and excessive daytime sleepiness, with impairment in important areas of functioning and quality of life. Current treatments, such as timed exposure to bright light and exogenous melatonin, primarily serve to align the timing of circadian rhythms or increase the strength of the circadian signal. Although these treatments are effective, their use in clinical practice has been limited by the availability of adequate diagnostic tools and large-scale randomized controlled clinical trials. The rapid advances in our understanding of the molecular, cellular, and physiologic basis of circadian regulation and the pathophysiology of these disorders will lead to more targeted and effective treatments.