Objective: To explore the association between bedroom light at night (LAN) exposure and body mass index (BMI) in children at 1 year follow up, so as to provide new strategies for obesity prevention. Methods: From December 2021 to May 2022, cluster random sampling was conducted, involving 648 children from two primary schools in Tianchang, Chuzhou City, Anhui Province, China, to assess bedroom LAN exposure of children during sleep. A questionnaire survey and physical examination were carried out in May 2022. Multivariate linear regression was performed to analyze the correlation between bedroom LAN exposure and BMI variable quantity at 1 year follow up (May, 2023). Results: The median intensity of bedroom LAN exposure during the sleep episode was [1.11(0.35,3.24)lx] in children. The proportion of the sample exposed to an average light intensity of ≥3 lx was 27.5%, while 19.0% was exposed to a LAN intensity of ≥5 lx during the sleep episode. In the multivariable linear regression, after adjusting for covariates, including sex, baseline age, sleep duration, family monthly income, and maternal education level, exposure to a 1 h-average post bedtime LAN intensity of ≥3 lx ( β=0.25, 95%CI =0.05-0.44) and LAN≥5 lx ( β=0.34, 95% CI = 0.12-0.55) was associated with a gain of 0.25 and 0.34 kg/m 2, respectively, in the children s BMI at the 1 year follow up ( P < 0.05). Conclusions A positive correlation was found between bedroom LAN exposure and BMI variable quantity at 1 year follow up in children. Thus, reduced bedroom LAN exposure might be useful for interventions aimed at obesity prevention.