Nightmares are dreams that evoke intense negative feelings. While occasional nightmares are common and generally harmless, frequent nightmares can be a clinical problem which should not be underestimated: Nightmares often co‐occur with psychological problems and may even increase risk for suicide. In the current thesis I will present empirical studies on epidemiology of nightmares among Finnish general adult population. These studies are based on series of FINRISK health surveys that have been conducted with random cross‐sectional population samples of Finnish adults aged 25‐74 years every five years since 1972. Surveys of 1972‐2012 are utilized in our studies with total N of 78 345. 

With FINRISK data, we have investigated the prevalence of nightmares, risk factors for nightmares and association between nightmares and suicide. FINRISK data also includes veterans of the Second World War and we have studied how nightmare frequency of these veterans differs from general population and is the association between nightmares and suicide different among veterans compared to nonveterans. Our results reveal that nightmares are not uncommon among this population: Around 4% of the participants report frequent nightmares and the prevalence is affected by gender and age of the participant. Women reported more nightmares than men and young participants reported less nightmares than those of advanced age. War veterans have elevated nightmare frequency, even over 30 years after the war has ended. Nightmares also have strong association with various other problems, insomnia and depression being the most prominent ones and frequent nightmares increase suicide risk slightly. 

However, as nightmares are dreams and as such their nature is that of a subjective experience, measuring them in epidemiological setting is quite challenging. Therefore, in addition to the empirical studies I will analyse what dreams and nightmares are, how can they be measured and what do retrospective questionnaire studies actually tell us about nightmares. This analysis is based on philosophical and theoretical tradition of the study of consciousness and our epidemiological results will be framed in a novel way based on this philosophical analysis.