Seasonal variations in sleep problems at latitude 63°-65° in Norway: The Nord-Trondelag Health Study, 1995-1997.

Most studies on seasonal variability in sleep have asked participants if they think their sleep quality varies with the seasons, which reveals the research hypothesis to the participants. To date, the hypothesis of seasonal variation in sleep has not been tested in a large population-based fully blinded study. The aim of the current study was to investigate monthly variations in sleep problems in a geographic region of Norway with large seasonal differences in daytime light. Using data from a general health survey, the authors had access to information on sleep in the general population, collected across the seasons over 2 years without linking sleep to seasonal variation. In all, 43,045 participants (mean age, 44.6 years) of the Nord-Trøndelag Health Study, 1995-1997 (referred to as "HUNT-2"), provided reports of insomnia symptoms and time in bed in all months except July. The mean prevalence of insomnia symptoms was 12.4%. No evidence of a seasonal variation on reports of insomnia symptoms or time in bed was found. These null findings are in marked contrast to previous seasonality studies of sleep. Previous studies reporting seasonal variations in sleep and insomnia might have been subject to publication biases and lack of blinding to the research hypothesis.