This chapter discusses the organization of human sleep by the brain mechanisms and specific sleep disorders that lead to disturbances in the brain mechanisms. Non-rapid eye movement (NREM) sleep is controlled by brainstem oscillators whose activation leads to the multiple physiological accompaniments of the NREM state. The dream is the unusual mental content that often accompanies REM sleep. As humans transition from wakefulness to sleep, characteristic physiological changes include decreases in respiratory rate, heart rate, and blood pressure. The domain of the insomnias benefits from thoughtful differential diagnosis, as the causation may be both multiple and obscure. The insomnia complaints that are comorbid with medical disorders include both sleep disturbances caused by medical symptoms, and also those sleep disturbances caused by the pathophysiology underlying the medical condition. Restless leg syndrome (RLS) and periodic limb movements of sleep (PLMS) are considered as sleep disorders.