To explore nonlinear effect of BMI and sleep duration on the risk of self-reported pain in Chinese elderly and explore their potential interactions.

The cohort study was conducted based on the Chinese health and retirement longitudinal study (CHARLS). Piecewise linear splines modeled nonlinear exposures, and competing risk models (Fine-Gray) analyzed associations between BMI, sleep duration, and self-reported pain, after adjusting for demographic characteristics, lifestyle factors, and health status with all-cause death as competing event.

Total of 4 409 elderly individuals were included, with a median follow-up of 3.1 years. During follow-up, 1 521 participants (34.50%) reported pain occurrence, and 199 (4.51%) died. Both underweight (BMI<20.0 kg/m²) and overweight/obese (BMI>26.9 kg/m²) were associated with a significantly increased short-term risk of pain (β=5.447, P<0.001; β=3.582, P<0.001), which gradually decreased over time (β=-1.436, P<0.001; β=-0.960, P<0.001). Similarly, short sleep (<6 hours) and long sleep (>9 hours) were associated with an elevated short-term pain risk (β=5.516, P<0.001; β=8.771, P<0.001), with the risk diminishing over time (β=-1.448, P<0.001; β=-2.354, P<0.001). Notably, long sleep duration exhibited the strongest effect. Interaction analysis revealed negative multiplicative interactions at baseline for underweight with short sleep (β=-1.953, P<0.001), overweight/obesity with short sleep (β=-1.326, P<0.001), and overweight/obesity with long sleep (β=-1.612, P<0.001), indicating that the short-term pain risk in these combined-risk groups was lower than the sum of their individual effects. However, time-interaction analysis showed that the pain risk in these combined-risk groups declined significantly slower over time compared to those with single risk factors (β=0.521, P<0.001; β=0.355, P<0.001; β=0.435, P<0.001), suggesting more persistent pain and slower recovery.

Abnormal BMI and abnormal sleep duration are significantly associated with an increased risk of pain requiring medication intervention in the Chinese elderly population, with long sleep duration showing the strongest association. Complex synergistic effects exist between abnormal weight and sleep problems: although combined-risk groups do not exhibit additive short-term pain risk, they experience prolonged pain duration and slower recovery, making them a high-risk population for pain chronification.