Focusing on disorders of initiating and maintaining sleep (insomnias), this presentation emphasizes the value of the differential diagnosis in sleep disturbance. Often, insomnia may be the presenting symptom in patients with another, sometimes more serious, condition. Specifically, insomnia has been associated with apnea, myoclonus, substance abuse, and neurologic, endocrine, cardiovascular, and gastrointestinal illnesses, as well as with psychiatric disturbances such as depression, schizophrenia, and obsessional disorders. The correlation of insomnia with increased mortality and morbidity further clarifies the need for accurate diagnosis.