Seasonality of cognitive function in the general population: the Rotterdam Study

Seasonal variation in cognitive function and underlying cerebral hemodynamics in humans has been suggested, but not consistently shown in previous studies. We assessed cognitive function in 10,276 participants from the population-based Rotterdam Study, aged 45 years and older without dementia, at baseline and at subsequent visits between 1999 and 2016. Seasonality of five cognitive test scores and of a summary measure of global cognition were determined, as well as of brain perfusion. Using linkage with medical records, we also examined whether a seasonal variation was present in clinical diagnoses of dementia. We found a seasonal variation of global cognition (0.05 standard deviations [95% confidence interval: 0.02–0.08]), the Stroop reading task, the Purdue Pegboard test, and of the delayed world learning test, with the best performance in summer months. In line with these findings, there were fewer dementia diagnoses of dementia in spring and summer than in winter and fall. We found no seasonal variation in brain perfusion. These findings support seasonality of cognition, albeit not explained by brain perfusion.

Sleep in the land of the midnight sun and polar night: The Tromsø study

While some diseases and human behaviors fluctuate consistently with season, the extent of seasonal variations in sleep, especially at high latitudes, is less consistent. We used data from a geographic region (69º North) with extremely large seasonal differences in daylight that had the participants blinded for the current study's hypotheses. Data were derived from the Tromsø Study (2015-2016), an ongoing population-based study in Northern Norway comprising citizens aged 40 years and older (n = 21,083, participation = 64.7%). The sleep parameters included bedtime, rise time, sleep onset latency (SOL), and total sleep time. Insomnia was defined according to recent diagnostic criteria (International Classification of Sleep Disorders; ICSD-3). We found some evidence of monthly or seasonal variation in sleep problems. Insomnia was most common during the winter months among men, but not women. No seasonal or monthly effects were observed for sleep duration. SOL was slightly longer during the winter months, but the differences were small and hardly of any clinical relevance. The small or non-existing seasonal variation in sleep and sleep difficulties indicate that extreme seasonal variation in daylight is of little influence on sleep status. The city of Tromsø is a modern city with considerable level of artificial light, which may contribute to the observed rather stabile sleep patterns throughout the year.

Sleep patterns and insomnia in a large population-based study of middle-aged and older adults: The Tromsø study 2015-2016

Epidemiological studies assessing adult sleep duration have yielded inconsistent findings and there are still large variations in estimation of insomnia prevalence according to the most recent diagnostic criteria. Our objective was to describe sleep patterns in a large population of middle-aged and older adults, by employing accurate measures of both sleep duration and insomnia. Data stem from the Tromsø Study (2015-2016), an ongoing population-based study in northern Norway comprising citizens aged 40 years and older (n = 21,083, attendance = 64.7%). Sleep parameters were reported separately for weekdays and weekends and included bedtime, rise time, sleep latency and total sleep time. Insomnia was defined according to recent diagnostic criteria (International Classification of Sleep Disorders; ICSD-3). The results show that 20% (95% confidence interval,19.4-20.6) fulfilled the inclusion criteria for insomnia. The prevalence was especially high among women (25%), for whom the prevalence also increased with age. For men, the prevalence was around 15% across all age groups. In all, 42% of the women reported sleeping <7 hr (mean sleep duration of 7:07 hr), whereas the corresponding proportion among males was 52% (mean sleep duration of 6:55 hr). We conclude that the proportion of middle-aged and older adults not getting the recommended amount of sleep is worryingly high, as is also the observed prevalence of insomnia. This warrants attention as a public health problem in this population.