Background Obese people tend to report more insomnia symptoms than their normal-weight counterparts. However, longitudinal studies are sparse. We aimed to examine whether relative weight is associated with changes over time in insomnia symptoms. Methods The data were derived from the prospective Helsinki Health Study cohort among 40–60-year-old employees of the City of Helsinki. Baseline survey data were collected in 2000–2002 (n = 8960, response rate 67%) and follow-up data in 2007 (n = 7332, response rate 83%). Body mass index (BMI) was calculated from self-reported height and weight. Insomnia symptoms were measured at baseline and follow-up. Multinomial logistic regression analysis was used. Results 45% of women and 59% of men were overweight or obese at baseline. Most participants reported at least occasional or transient insomnia symptoms, with 29% of women and 34% of men reporting no or rare symptoms at both time points. Adjusting for age, baseline BMI was strongly associated with persistent and increasing insomnia symptoms. The associations mainly remained after adjustments for marital status, occupational class, alcohol consumption, physical inactivity, common mental disorders, physical health, and employment status. Conclusion BMI is a strong determinant of persistent and increasing insomnia symptoms. Successful maintenance of a healthy body weight probably helps prevent insomnia symptoms, and their increase and persistence, among both women and men.
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