This chapter discusses circadian rhythms and the known factors which influence them as well as provide an update on the evaluation and treatments of the six circadian rhythm sleep disorders. The six circadian rhythm sleep disorders include delayed sleep phase type, advanced sleep phase type, non-24-hour sleep/wake syndrome, irregular sleep/wake rhythm, shift work sleep disorder, and jet lag disorder. Zeitgebers such as light are entraining agents of the circadian system and the manipulation of zeitgebers can be used as a therapeutic tool for the circadian rhythm sleep disorders. Measurements of circadian phase can be obtained through various surrogate markers of circadian rhythms, including the core body temperature minimum and the dim light melatonin onset (DLMO). Circadian rhythm sleep disorders are commonly under recognized in clinical practice and should be a part of the differential diagnosis of people who present with symptoms of insomnia and daytime sleepiness.