Insomnia is defined as difficulty initiating or maintaining sleep, or both, despite adequate opportunity and time to sleep, leading to impaired daytime functioning. Insomnia may be a cause of or result of poor quality and/or quantity of sleep. Insomnia is very common. Ninety percent of the general population has experienced acute insomnia at least once. Approximately 10% of the population may suffer from chronic (long-standing) insomnia.