Background: Short sleep is increasingly recognized as a risk factor for obesity. Current evidence has however been limited to cross-sectional studies or longitudinal studies using self-reported sleep. We explored the directionality of the association between objectively measured sleep and body mass index (BMI).
Methods: The study consists of 1,031 participants from the general population (52% women, 45-91 years at baseline). Sleep, BMI and waist circumference (WC) were measured twice across a follow-up of 6 years. BMI and WC were measured at the research center. Total sleep time (TST, hrs), sleep onset latency (SOL, min), sleep efficiency (SE, %) and wake after sleep onset (WASO, min) were estimated by a wrist-worn actigraph. Cross-sectional and longitudinal associations in both directions were explored.
Results: Cross-sectionally, an hour shorter TST was associated with approximately 0.5kg/m2 higher BMI. Longitudinally, longer TST and higher SE were associated with lower BMI (βTST= -0.75, 95%CI:-1.08,-0.42; βSE= -0.04, 95%CI:-0.08,-0.01). Conversely, one kg/m2 higher BMI was prospectively associated with 0.02 hours shorter TST (95%CI: -0.03,-0.01), and this association was more pronounced over time. Results from analyses with WC were in line with those of BMI.
Conclusions: This is the first study to explore bidirectionality in the association between objectively measured sleep and BMI in a large population of middle-aged and older adults. Indices of poor sleep were associated with higher and less stable BMI across time. Conversely, a high BMI was associated with a decrease in sleep duration. This confirms that the relation between sleep and body size is bidirectional, and changes in either sleep or BMI are likely to co-occur with changes in health through multiple pathways.