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Previous studies on the prevalence of sleep disturbances have shown that insomnia occurs in 3.2-42% of different populations. The wide reported variation in prevalence prompted a rigorous definition of insomnia to be introduced in this study. Randomly selected members of the population aged 30 to 65 years from two geographically different rural parts of central Sweden answered a sleep questionnaire. The response rates were 69.2% and 70.2%, respectively. Females significantly more often reported difficulty in falling asleep (7.1% of the women and 5.1% of the men). Among women 8.9 and among men 7.7% of individuals reported trouble with nocturnal awakenings. Using a stringently defined concept of insomnia as a disorder of initiating sleep (DIS), the prevalence rate of insomnia among women was 1.1% and among men 0.5%. Defining insomnia as a disorder of maintaining sleep (DMS), the prevalence among both women and men was 1.1%. Defining insomnia as a disorder of initiating and maintaining sleep (DIMS), the prevalence rate was 1.7% among women and 1.4% among men. This prevalence, which is lower than previously reported, demonstrate the importance of an operational definition of insomnia.