The majority of patients with circadian rhythm sleep disorders (CRSD) report that the disorder typically began in early childhood or adolescence. The diagnosis of CRSDs can be established through three procedures: clinical interview, longitudinal monitoring of the sleep-wake cycle, and assessment of biological markers of the internal circadian rhythmicity. Bright light therapy and melatonin treatment, or a combination of these two, have proved to be the most effective treatment modalities for patients with CRSDs. This chapter reviews the disorders that have been systematically examined with regard to CRSDs, specifically mood disorders, attention-deficit/hyperactivity disorder (ADHD), personality disorders, and obsessive-compulsive disorder (OCD). Several cases of disrupted sleep-wake schedule as iatrogenic effects of psychoactive drugs are documented in the literature. Switching treatment to the atypical neuroleptic clozapine established a more organized and stable sleep-wake pattern and improved the clinical state of the patient.