Self-reported seasonality is associated with complaints of sleep problems and deficient sleep duration: the Hordaland Health Study.

Change in sleep duration dependent on time of year is a central characteristic of seasonal affective disorder (SAD). In a community health survey, we analysed associations between seasonality, subjective sleep problems and sleep duration. Totally, 8860 subjects (3531 men and 5329 women) aged between 40 and 44 years were included in the study. Seasonal changes in mood and behaviour were measured by the Global Seasonality Score (GSS) questionnaire, and subjects were grouped in high (GSS > or = 11), moderate (GSS 8-10) or low (GSS < 8) seasonality groups. Sleep symptomatology was assessed using a modified version of the Karolinska Sleep Questionnaire. Significant sleep duration deficiency was defined as the difference between subjective sleep need and sleep duration of at least 1 h. Sleep problems suggesting insomnia as well as increased daytime sleepiness were more prevalent in the high/moderate seasonality groups compared with the low seasonality group. Seasonality was furthermore associated with shorter sleep duration and increased subjective sleep need. Significant sleep duration deficiency was more prevalent in subjects reporting high (men 20% and women 21%) and moderate (men 13% and women 19%) seasonality than subjects reporting low (men 10% and women 14%) seasonality. In conclusion, we found seasonal changes in mood and behaviour to be associated with several sleep-related complaints. Sleep duration deficiency increased with increasing seasonality, mainly due to increasing subjective sleep need.