Circadian rhythms are the endogenous near-24-h oscillations in physiologic processes. In mammals the suprachiasmatic nucleus serves as the primary circadian pacemaker, and it maintains rhythmicity at a genetic level through a complex transcription-translation feedback loop of core circadian clock genes. The circadian clock is entrained to the environment through daily exposure to light and melatonin. Disruption of these endogenous rhythms or the ability to entrain to the surrounding environment results in the circadian rhythm sleep–wake disorders (CRSWDs). Patients with CRSWDs can present with either late sleep/wake times (delayed sleep–wake phase disorder), early sleep/wake times (advanced sleep–wake phase disorder), inconsistent sleep/wake times (irregular sleep–wake rhythm disorder) or sleep–wake times that move progressively later each day (non-24-h sleep–wake rhythm disorder). Diagnosis of these disorders relies on the use of sleep logs and/or actigraphy to demonstrate the daily patterns of rest and activity. Treatment of the CRSWDs focuses on sleep hygiene and strategically timed light and melatonin.