**Abstract**

Circadian rhythm disorders are a group of sleep conditions that involve a misalignment of an individual's internal timekeeping system with that of one's desired sleep-wake time. This desynchrony can compromise sleep health as well as the functioning of other organ system, and significantly diminish one's quality of life. There are six well-defined circadian rhythm disorders that can be classified as either intrinsic or extrinsic, based on the underlying factors that contribute to the condition. Intrinsic circadian disorders include the following: 1) advanced sleepwake phase disorder, 2) delayed sleep-wake phase disorder, 3) irregular sleep-wake rhythm disorder, and 4) non-24-hour sleep-wake rhythm disorder. The two circadian disorders caused by external factors include 1) shift work disorder, and 2) jet lag disorder, both of which are due to behaviorally mediated misalignments of circadian system. This chapter serves to summarize these disorders, guide clinicians towards screening and evaluation of these conditions, and introduce basic treatment strategies that can be applied by non-sleep medicine clinicians.

**Keywords:** circadian rhythm disorder, advanced sleep–wake phase disorder, delayed sleep–wake phase disorder, irregular sleep–wake rhythm disorder, non-24-hour sleep–wake rhythm disorder, shift work disorder, jet lag disorder, melatonin, phototherapy
