Revealing the Research Hypothesis to Subjects

people there report no difference in insomnia between winter, when there are only 3 hours of daylight, or in summer, when there are 21 hours of daylight.

The researchers were surprised because earlier studies have indicated that darkness
during winter might accentuate depression and the sleeplessness that derives from it.

Seasonal Variations in Sleep Problems at Latitude 63°–65° in Norway The Nord-Trøndelag 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.

This doesn’t mean SAD doesn’t exist. But if you can’t find seasonal sleep variation in 40,000 people, it probably doesn’t exist. If prolonged sunlight during long extreme-north summer days makes it harder to sleep normally, then people must have adjusted (e.g. blinds that actually block the light).