Nocturnal Bright Light Exposure Linked to Early Pubertal Onset

Exposure to light at night (LAN) may accelerate the onset of puberty among both boys and girls, with the most potent effects observed during the post-bedtime period, according to study findings published in The Journal of Clinical Endocrinology & Metabolism.

Along with recent US data indicating a rise in early menarche, trends in earlier puberty among Chinese youth may have significant public health implications. Emerging evidence suggests that LAN, an environmental endocrine disruptor, could contribute to this shift. However, research on the relationship between LAN and timing of pubertal onset remains limited.

In this prospective longitudinal study, researchers aimed to examine the link between personal bedroom LAN exposure and earlier pubertal onset among both boys and girls.

The study randomly enrolled 886 children, boys (n=474) and girls (n=412) aged 6 to 10 years from the Tianchang Elementary School Students Light at Night (TESS-LAN) study, an ongoing, longitudinal trial of students from 2 selected primary schools in eastern China between September and December 2022.

Our study provides new evidence from a prospective cohort that increased exposure to bedroom LAN, particularly during the post-sleep period, is significantly associated with earlier onset of breast development in girls and testicular development in boys.

Each participant measured bedroom LAN exposure for 2 consecutive school nights using a calibrated photometer placed beside the pillow at eye level. Parents recorded sleep and wake times in a sleep diary and verified correct device placement through uploaded photos. LAN exposure was analyzed by quartiles and thresholds of at least 3 lx and at least 5 lx, with additional data on sleep habits and perceived bedroom darkness collected through questionnaires.

Bedroom LAN measurements showed a median (IQR) exposure of 1.3 (0.5-3.8) lx, with 276 participants (31.2%) experiencing optimal LAN levels of at least 3 lx for a mean 32.8 minutes. Post-bedtime light exposure was significantly higher among girls than boys (P <.05).

Boys in the highest LAN exposure group experienced puberty onset 3.84 months earlier (time ratio [TR], 0.96; 95% CI, 0.95-0.98), while girls experienced it 4.12 months earlier (TR, 0.96; 95% CI, 0.94-0.98), compared with those in the lowest exposure group.

Each additional 30 minutes of LAN exposure of at least 3 lx was linked to a 12% higher risk for earlier thelarche among girls (hazard ratio [HR], 1.12; 95% CI, 1.07-1.16) and a 9% higher risk for earlier testicular development among boys (HR, 1.09; 95% CI, 1.02-1.16). The association was strongest for post-bedtime light exposure compared with pre-awake light exposure.

Boys with early pubertal onset had higher median bedroom LAN exposure (3.4 lx vs 1.1 lx; P <.001). These differences were most evident during post-sleep and pre-awakening periods, with girls in the early-onset group also showing significantly higher nighttime light intensity and duration during sleep.

Key study limitations include the short-term measurement of bedroom light exposure confined to sleep periods; potential unmeasured confounding factors, such as genetics and diet; and limited generalizability due to sampling from a single city in China.

The study author authors concluded, “Our study provides new evidence from a prospective cohort that increased exposure to bedroom LAN, particularly during the post-sleep period, is significantly associated with earlier onset of breast development in girls and testicular development in boys.”

This article originally appeared on Endocrinology Advisor