Associations between circadian factors and noncommunicable diseases: a populationbased study in Finland

The aim of this thesis was to study the association between circadian factors and common noncommunicable diseases. To our knowledge, these associations have not been investigated before in a large population-based cohort. The study of these associations is important as the global morbidity and mortality rates for noncommunicable diseases are increasing. Thus, there is a need to better understand the etiology of these diseases. The data to conduct the four studies included into this thesis was derived from The National FINRISK 2012 Study. A random sample of 10,000 Finns aged 25 to 74 from five different geographical areas in Finland were invited to participate in the study. The overall participation rate was 64%. The participants reported their sleeping habits and disturbances, seasonal variations in mood and behavior as well as their circadian preference to the daily activities. As a part of their health examination, the participants were also asked about their diagnoses or treatments they had received for a set of common noncommunicable diseases and medical conditions during the past 12 months. Here, the results are first presented based on a single-item (study I–III) and the final analysis (study IV) with regression analyses controlled for covariates. According to our results, the average duration of self-reported sleep in the Finnish working-age population has declined as compared to previous years. Now, 63% of the population is sleeping on average 7-8 hours daily, and the population is distributed into evening types (13.5%), intermediate types (42.7%), and morning types (43.8%). Seasonal variations in sleep duration, social activity, mood and energy level, as well as in weight and appetite are rather common. As hypothesized, circadian factors were associated with several noncommunicable diseases. The odds for evening chronotype for depression and other psychiatric illnesses was significant in single-item analysis and remained significant in the final analysis. Higher seasonality and more frequent sleep problems were reported throughout the studies I–IV. Of all the circadian factors, sleep quality was the most sensitive probe yielding associations with noncommunicable diseases. Insufficient and poor sleep, eveningness and seasonal variations are common among Finnish adult population and associated with the increased odds for several noncommunicable diseases. By better recognizing those with the increased risk to develop noncommunicable diseases, we could improve the assessment of health status, health care practices and health policies.