Insomnia (or sleeplessness) is most often defined by an individual's report of sleeping difficulties. While the term is sometimes used in sleep literature to describe a disorder demonstrated by polysomnographic evidence of disturbed sleep, insomnia is often defined as a positive response to either of two questions: "Do you experience difficulty sleeping?" or "Do you have difficulty falling or staying asleep?"

Thus, insomnia is most often thought of as both a sign and a symptom that can accompany several sleep, medical, and psychiatric disorders, characterized by persistent difficulty falling asleep and/or staying asleep or sleep of poor quality. Insomnia is typically followed by functional impairment while awake. One definition of insomnia is difficulties initiating and/or maintaining sleep, or nonrestorative sleep, associated with impairments of daytime functioning or marked distress for more than 1 month."

Insomnia can be grouped into primary and secondary, or comorbid, insomnia. Primary insomnia is a sleep disorder not attributable to a medical, psychiatric, or environmental cause. A complete diagnosis will differentiate between:

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