Insomnia is the common sleep disorder, with prevalence estimates ranging from 10 to 20% of the population. An insomnia disorder is a syndrome consisting of at least one insomnia complaint in an individual with adequate circumstances for sleep, combined with daytime impairment. Given that the underlying causes of insomnia are often complex and multifactorial and that co-morbid insomnias are the most prevalent type of insomnia disorder, carefully differentiating between insomnia symptoms and other co-morbid conditions that may contribute to or be exacerbated by the insomnia symptoms, is critical for optimizing treatment efforts. The experience of transient insomnia symptoms at some point during the life course is nearly universal experience. A number of demographic, psychosocial, and behavioral risk factors have been identified as important risk factors for chronic insomnia. The physiological model of insomnia is based on the central assumption that insomnia represents a disorder of increased physiological arousal.