Association of residential greenness and sleep duration in adults: A prospective cohort study in China

Background: Exposure to residential greenness has been linked with improved sleep duration; however, longitudinal evidence is limited, and the potential mediating effect of ambient fine particulate matter (PM2.5) has yet to be assessed. Methods: We obtained data for 19,567 participants across seven counties in a prospective cohort in Ningbo, China. Greenness was estimated using Normalized Difference Vegetation Index (NDVI) within 250-m, 500-m and 1000-m buffer zones, while yearly average PM 2.5 concentrations were measured using validated land-use regression models, both based on individual residential addresses. Sleep duration was assessed using structured questionnaires at baseline and during follow-up. The longitudinal associations between residential greenness and sleep duration were evaluated using linear mixed-effect models. Adjustments for PM 2.5 and county were implemented. To identify the effect of greenness exposure on age-related declines in sleep duration, we included an interaction term between NDVI and visit (baseline and follow-up). Additionally, half-longitudinal mediation analyses were conducted to evaluate the potential mediating role of PM 2.5 in this relationship. Results: Each interquartile range (IQR) increase in NDVI within 250 m, 500 m and 1000 m was associated with increases in sleep duration of 0.044 h (95% CI: 0.028,0.061), 0.045 h (95% CI: 0.028,0.062), and 0.031 h (95% CI: 0.013,0.049), respectively. Associations were attenuated after adjusting for PM2.5. Farmers, homemakers, and short-nap individuals benefited the most from greenness exposure. Higher greenness exposure significantly lowered PM 2.5 levels, which was associated with a slower decline in sleep duration over the follow-up period. Conclusion: Exposure to higher levels of residential greenness was associated with increased sleep duration and a slower decline in sleep over time. County-level heterogeneity in the effects of residential greenness on sleep duration was observed. PM 2.5 partially mediating this relationship.