Seasonal variations in sleep duration and sleep complaints: A Swedish cohort study in middle-aged and older individuals.

Subjective sleep reports are widely used research tools in epidemiology. Whether sleep reports can differ between seasons is less clear. Using multivariable binary or multinomial logistic regression analyses, in the present Swedish cross-sectional two-centre cohort study (N = 19,254; mean age 61 years), we found that participants surveyed during the summer (June-August) were more likely to report short sleep duration (defined as ≤ 6 hr) compared with those interviewed during the autumn (odds ratio [95% confidence interval] = 1.14 [1.04-1.25]). Individuals interviewed in the winter (December-February) were less likely to report early awakenings compared with participants surveyed in the autumn (September-November; odds ratio [95% confidence interval] = 0.85 [0.75-0.96]). Complaints of difficulties in falling asleep and disturbed sleep were less common among participants interviewed during spring (March-May) compared with those interviewed during the autumn (odds ratio [95% confidence interval] = 0.86 [0.74-0.99] and 0.88 [0.79-0.98], respectively). No seasonal variations in reports of long sleep, difficulty maintaining sleep, or feeling not rested after sleep were observed. Additional subgroup analysis revealed that summer participants were more likely to report short sleep duration and early morning awakenings than individuals surveyed in winter. In conclusion, this Swedish study indicates that self-reported sleep characteristics may vary across seasons. Further studies are needed to confirm our findings.