Occasional sleep difficulties are fairly common in adults. Therefore, insomnia disorder is diagnosed only when the sleep difficulty occurs at least three nights per week for a period of three months or longer. Insomnia disorder often co-occurs with medical conditions and other mental disorders. However, insomnia disorder should only be diagnosed if the insomnia is not adequately explained by the co-occurring condition and if the insomnia is severe enough to warrant independent clinical attention. Finally, insomnia disorder is diagnosed only if the individual is significantly distressed or impaired by the insomnia or resulting daytime symptoms.