Objectively measured sleep and body mass index: a prospective bidirectional study in middle-aged and older adults

In recent years, short sleep has been increasingly recognized as a risk factor for obesity. However, current evidence has so far been limited to cross-sectional studies or longitudinal studies using self-reported sleep. Therefore, we explored the directionality of the association between objectively measured sleep and body mass index (BMI).

The study consists of 1031 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 six 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. In addition, cross-sectional and longitudinal associations in both directions were explored.

An hour shorter TST was cross-sectionally associated with approximately 0.5 kg/m² higher BMI. Longitudinally, longer TST and higher SE were associated with lower BMI (β_TST = -0.75, 95% CI: -1.08, -0.42; β_SE = -0.04, 95% CI: -0.08, -0.01). Conversely, one kg/m² higher BMI was prospectively associated with 0.02 h 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.

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.