Circadian rhythm sleep disorders are common conditions manifested as misalignment between the sleep period and the physical/social 24-h environmental cycle. Delayed sleep phase (typical in adolescents) and advanced sleep phase (frequent in the elderly), situations in which the sleep period is displaced to a later or earlier time, respectively, are the two most prevalent circadian rhythm sleep disorders. There are, however, nine possible diagnoses of clinical interest. Since light is the main cue used in synchronizing the biological clock, blind individuals and night-shift/rotating-shift workers are more prone to develop circadian rhythm sleep disorders. In this article, the circadian rhythm syndromes included in the new International Classification of Sleep Disorders are reviewed as well as the most recent guidelines for diagnosis and treatment.