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Estimates of the prevalence of insomnia vary as a function of the specific diagnostic questions asked of the patient, as well as the duration and severity criteria used in the studies to define a case of insomnia. Prevalence estimates range from 10% to 50% of the adult population. The lower prevalence rates are derived from studies that require the insomnia to be chronic and to be associated with daytime consequences. For example, one study1 examined the 6-month prevalence of "trouble falling asleep, trouble staying asleep, or waking up too early."