Circadian rhythm sleep disorders (CRSDs) cause disturbances in sleep and wakefulness due to a misalignment between the timing of the body’s intrinsic circadian clock and environmental light and social activity cycles. This chapter reviews these disorders with an emphasis on their neural pathways, genetic mechanisms, and regulatory factors. The authors discuss the relationship of CSRDs to physical and mental health, the treatment of CRSDs with circadian rhythm adjustment techniques, and the relationship of CSRDs to psychiatric disorders, along with potential chronobiologic treatments of psychiatric disorders. The chapter specifically addresses delayed sleep phase disorder, advanced sleep phase disorder, non-24-hour sleep–wake rhythm disorder, irregular sleep–wake disorder, shift work disorder, and chronobiology and psychiatric disorders. Melatonin and bright light therapy are covered.