Bright Light Exposure Is Positively Associated with Serum Testosterone in Adult Men: A National Cross-sectional Analysis.
Plain-language summary
This study set out to see whether exposure to bright light is linked to testosterone in men, an area far less studied than diet and exercise. It analyzed data from over 2,000 adult men in the US, using wrist-worn devices to measure daily bright light (over 2,000 lux) against morning blood testosterone, adjusting for age, weight and activity. More bright light, particularly in the afternoon, went with higher testosterone, and the link was strongest in the men who got the least light overall. The design cannot show cause and effect and it did not separate indoor bright light from sunlight. The same research group published a companion analysis in this library using reported time spent outdoors instead of measured light, so the two are one team measuring one question two ways rather than two independent findings.
Key findings
- More bright light, over 2,000 lux, went with higher testosterone.
- Only afternoon exposure was associated with testosterone. Morning and evening exposure showed no significant relationship.
- The link was strongest in the men who got the least light overall.
How to analyze the study yourself
- Study type
- Cross-sectional studies measure exposure and outcome at the same time, so they can show associations but not prove what caused them.
- Population
- Adult men from the US National Health and Nutrition Examination Survey (NHANES).
- Funding
- Not reported
- Takeaway
- This study found that men with more bright light exposure had higher testosterone, and that the association came from afternoon light alone, with morning and evening showing no relationship. If you want to act on it, the afternoon is the part of the day this study points at, particularly if you are indoors for most of it.