Insomnia is a symptom[1] which can accompany several sleep, medical and psychiatric disorders, characterized by persistent difficulty falling asleep and/or staying asleep despite the opportunity. Insomnia is typically followed by functional impairment while awake. Both organic and non-organic insomnia without other cause constitute a sleep disorder, primary insomnia.[2] 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."[3]

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.[4] Insomnia is 1.4 times more common in women than in men.