Insomnia is a symptom of any of several sleep disorders, characterized by persistent difficulty falling asleep or staying asleep despite the opportunity. Insomnia is a symptom, not a stand-alone diagnosis or a disease. By definition, insomnia is "difficulty initiating or maintaining sleep, or both" and it may be due to inadequate quality or quantity of sleep. It is typically followed by functional impairment while awake. Both organic and non-organic insomnia without other cause constitute a sleep disorder, primary insomnia.

According to the United States Department of Health and Human Services in the year 2007, approximately 64 million Americans regularly suffer from insomnia each year. Insomnia is 1.4 times more common in women than in men.

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