Insomnia is a symptom of difficulty initiating and maintaining sleep or experiencing nonrefreshing sleep and is associated with daytime consequences. Although insomnia is typically secondary to a medical, psychiatric, circadian, or sleep disorder, it can also be a primary disorder. Primary insomnia is estimated to occur in 25% of all chronic insomnia patients. It is hypothesized to be a disorder of hyperarousal, which has been supported by research on the autonomic nervous system and hypothalamic-pituitary-adrenal axis function. Chronic insomnia is prevalent in 10% of the adult population. Age, sex, medical and psychiatric disease, and shift work all represent an increased risk of chronic insomnia. The morbidity of insomnia varies as a function of etiology. While transient insomnia produces sleepiness and impairment in psychomotor performance, chronic insomnia is associated with absenteeism, frequent accidents, memory impairment, and greater health care utilization. The most consistent impact of insomnia is a high risk of depression.