Brighter bedroom light at night predicts risk for earlier pubertal onset: a two-year longitudinal study.

Context: Light at night (LAN), as an environmental endocrine disruptor, can accelerate gonadal development in animals, but evidence in humans remains limited.

Objective: To investigate whether individual-level bedroom LAN exposure is associated with accelerated pubertal onset in boys and girls.

Methods: This prospective study was performed from September 2022 in Tianchang, Anhui province, China, investigated 886 boys and girls aged 6 to 10 years. Bedroom LAN exposure was recorded at one-minute intervals for two nights using a portable illuminance meter at baseline. Breast development for girls and testicular volume for boys were assessed at baseline and every 6 months for 2 years. Accelerated failure time (AFT) models were utilized to examine the associations between timing, intensity and duration of LAN exposure with earlier onset of puberty.

Results: Puberty started 3.84-month earlier among boys (TR, 0.96; 95%CI, 0.95-0.98) and 4.12-month earlier among girls (TR, 0.96; 95% CI, 0.94-0.98) in the highest LAN exposure group compared to the lowest exposure group. Each 30-minute increase in the duration of LAN exposure ≥ 3 lx was associated with an 9% higher risk of earlier testicular development in boys (HR, 1.09; 95% CI, 1.02-1.16), and a 12% higher risk of earlier thelarche in girls (HR, 1.12; 95% CI, 1.07-1.16). Notably, the increased risk was more pronounced with post-bedtime light (PBL) exposure compared to pre-awake light (PAL) exposure.

Discussion: These findings suggest an accelerating effect of individual-level LAN exposure on pubertal onset in both boys and girls, particularly during the post-bedtime period.