U-shaped association between outdoor artificial light at night exposure and metabolic syndrome in Chinese youth: Findings from a nationally representative study.

Artificial light at night (ALAN) has emerged as a potential environmental risk factor, yet evidence on its association with metabolic syndrome (MetS) in pediatric populations remains unclear. We conducted a nationally representative study in 2013, recruiting 14,804 children aged 7-17 years from 94 schools in seven Chinese provinces/municipalities. Outdoor ALAN exposure was quantified using 3-year average calibrated satellite-derived nighttime light data around each school. MetS was defined by the Chinese pediatric criteria. Mixed-effects logistic regression and restricted cubic spline (RCS) models were used to examine the linear and non-linear associations between ALAN and MetS, as well as its components. Two-piecewise models estimated associations across thresholds. The mean age of the participants was 11.89 years, and 51.24 % were boys. The overall prevalence of MetS was 4.03 %. Higher ALAN exposure was significantly associated with an increased prevalence of MetS (odds ratio [OR] = 1.104, 95 % confidence interval [CI] = 1.019-1.195), high blood pressure (OR = 1.120, 95 % CI = 1.066-1.178), and abdominal obesity (OR = 1.075, 95 % CI = 1.033-1.119). RCS revealed U-shaped associations between ALAN and MetS, as well as high blood pressure, with thresholds at 15.933 and 26.868 nW/cm2/sr, respectively. In contrast, no significant association was found with elevated fasting blood glucose or high triglycerides (all P for non-linear >0.05). Stratified analyses revealed stronger associations among boys, urban residents, and participants who were physically inactive. These findings underscore that both insufficient and excessive ALAN may be detrimental, highlighting the importance of maintaining an optimal level of nighttime light exposure and promoting physical activity to protect youth metabolic health.