Sleep difficulties are common in the adult population, but very few studies have evaluated their persistence and have focused mainly on insomnia. Sleep disorders are health conditions diagnosed by the physician, whereas sleep difficulties are more a case of symptoms experienced by the patient. Especially the research data on the sleep difficulties of the working-age population are incomplete. Although it is known that lifestyle can affect sleep, some of the related information is contradictory. Limited information exists on how lifestyle impacts the persistence or course of sleep difficulties. However, a healthy lifestyle and positive changes are key to preventing sleep difficulties and related diseases. Sleep difficulties can be evaluated in several different ways but the most often used tools are easy, free of charge questionnaires.

The first goal of this study was to assess the persistence of sleep difficulties in working-age adults and the impact of lifestyle on them. The second goal was to identify how lifestyle changes affect the development of sleep difficulties. The third goal was to evaluate the reliability and validity of the Jenkins Sleep Scale in the studied sample. The research data consisted of the material of a prospective ongoing Finnish Public Sector survey based study. The number of participants in the study varied from 38,400 to 81,136. The percentage of women were between 80% to 83% and the average age of the participants were 44.7 to 52.1 years.

The follow-up study in question is still ongoing and the material for this dissertation consisted of 16 years of follow-up data. The Jenkins Sleep Scale served as a screening questionnaire for sleep difficulties, and its psychometric properties were investigated as part of the study in order to ensure the reliability of the scale.

The study shows that 60%-90% of sleep difficulties in working-age adults are persistent, with severe cases being particularly long-lasting. Lifestyle factors can affect the onset and alleviation of sleep difficulties, but established issues are hard to reverse. In addition, the Jenkins Sleep Scale is a questionnaire with robust psychometric properties, showcasing reliability in studies conducted with the working-age population. In the future focus should be on preventing the development of sleep difficulties, because recovery from them varies, and may even be unlikely.