Prevalence of insomnia in the adult Norwegian population.

A representative adult sample (18 years and above) of the Norwegian population, comprising 2001 subjects, participated in telephone interviews, focusing on the one-month point prevalence of insomnia and use of prescribed hypnotics. Employment of DSM-IV inclusion criteria of insomnia yielded a prevalence rate of 11.7%. Logistic regression analysis performed on the different insomnia symptoms revealed that somatic and psychiatric health were the strongest predictors of insomnia, whereas gender, age, and socioeconomic status showed a more inconsistent relationship. Use of prescribed hypnotic drugs was reported by 6.9% and was related to being female, elderly, and having somatic and emotional problems. Sleep onset problems and daytime impairment were more common during winter compared to summer. Use of hypnotics was more common in the southern (rather than the northern) regions of Norway. For sleep onset problems a Season x Region interaction was found, indicating that the prevalence of sleep onset problems increased in southern Norway from summer to winter, while the opposite pattern was found in the northern regions. The importance of clinically adequate criteria and seasonal variation in the evaluation of insomnia is briefly discussed.