Bright Light Exposure Is Positively Associated with Serum Testosterone in Adult Men: A National Cross-sectional...

Although lifestyle factors influencing testosterone, such as exercise and diet are well-studied, environmental factors like light exposure remain underexplored. We investigated whether bright light exposure (BLE) is associated with serum testosterone.

Adult men were extracted from the 2011-2014 National Health and Nutrition Examination Survey (NHANES) database. BLE (>2000 lux) was measured in average minutes per day (MPD) using wrist-worn accelerometers with ambient light sensors over 7-9 d. Survey-weighted multivariable linear regression identified factors associated with morning serum testosterone, adjusting for age, education, income, body mass index, physical activity, diabetes, vitamin D, dietary habits, and total daily light exposure. We additionally examined whether BLE timing-morning (6-11 am), afternoon (12-5 pm), or evening (5-10 pm)-was associated with testosterone.

Overall, 2209 men were included. Median age, testosterone, and BLE were 45 yr, 434 ng/dL, and 21.4 MPD, respectively. BLE was significantly associated with higher testosterone (β = 15.16 ng/dL per 10 min, p = 0.008). Afternoon exposure demonstrated the strongest association (β = 16.72, p = 0.006); morning and evening were not significant. A negative interaction between bright light and total daily light exposure (β = -0.14 per 10,000 daily total lux, p = 0.041) suggested the association was strongest in individuals with limited baseline light exposure. Limitations include the cross-sectional design, which precludes causal inference and raises the possibility of reverse causality, and reliance on a 2000 lux threshold that cannot distinguish indoor bright environments from direct sunlight.

This is the first study to demonstrate that BLE is independently associated with serum testosterone in adult men, with afternoon exposure showing the strongest association.