Daytime, nighttime, and day-night compound heatwaves and accelerated biological ageing: national cross-sectional and longitudinal evidence in China.

BACKGROUND

Despite emerging evidence linking heat exposure to accelerated biological aging, the potentially differential nexuses between daytime, nighttime and day-night compound heatwaves remain extensively unstudied in low- and middle-income populations. This study aimed to examine the associations of daytime, nighttime, and day-night compound heatwaves with biological age acceleration (BAA) in a Chinese national cohort.

METHODS

Based on the national cohort of adults aged 45+ years enrolled from 125 prefectural cities across China, we conducted parallel analyses using both cross-sectional (n = 8223) and longitudinal (n = 5047) designs. Biological age was assessed using the Klemera-Doubal method by collecting 11 clinical biomarkers at the baseline (2011) and follow-up survey (2015). Heatwaves were classified into daytime, nighttime, and day-night compound subtypes defined by daily maximum and minimum temperatures based on multiple percentile-based thresholds (P92.5-P97.5) and consecutive durations (2-4 days). Linear mixed-effects models were used to estimate the change in BAA (β, years) and its 95% confidence interval (CI) associated with per 1-day increase in cumulative heatwave exposure. Subgroup and sensitivity analyses were performed to check the robustness of heatwave-BAA associations in diverse populations and across heatwave definitions.

RESULTS

Prolonged heatwave exposure was consistently associated with greater BAA in both cross-sectional and longitudinal analyses. Under the P95-3d definition, nighttime heatwaves showed the largest current-year BAA estimates in both cross-sectional analyses (β = 0.0206, 95% CI: 0.0139 to 0.0273) and longitudinal analyses (β = 0.0147, 95% CI: 0.0094 to 0.0200). Day-night compound heatwaves were also positively associated with BAA in the current-year exposure window (cross-sectional: β = 0.0091, 95% CI: 0.0024 to 0.0158; longitudinal: β = 0.0145, 95% CI: 0.0084 to 0.0206) and showed more persistent associations across extended exposure windows. By contrast, daytime heatwaves showed weaker and less consistent associations with BAA, especially in cross-sectional analyses. Restricted spline function analyses supported the linear and threshold-free exposure-response associations, and subgroup analyses suggested stronger heatwave-related BAA effects among women and urban residents. Results were broadly robust across alternative heatwave definitions.

CONCLUSIONS

Cross-sectional and longitudinal analyses provided parallel evidence that nighttime and day-night compound heatwaves were more strongly associated with greater BAA than daytime heatwaves. Ageing-sensitive heat-health adaptation should incorporate nocturnal and compound heat risks into surveillance, early warning, and targeted protection for mid-to-older-aged adults.