Overview
- The study, published June 24 in General Psychiatry, followed 87,577 UK Biobank participants who wore wrist devices for seven days and found that average daytime exposure above 1,000 lux was associated with a 16% lower risk of dementia over a median 8.1 years of follow-up.
- Extended time in very bright light was also protective: spending about 0.70 hours per day at or above 5,000 lux was linked with an 17% risk reduction (HR 0.83) and less than 0.7 hours of bright light was a stronger predictor of future dementia than six established risk factors.
- Exploratory mediation analyses implicated circadian rest–activity rhythms and preservation of specific brain structures as explaining up to roughly one-third of the association, while vitamin D showed no mediating role.
- Protective associations were larger in higher-risk groups, including evening chronotypes, people with high nighttime light exposure, and APOE ε4 carriers, with subgroup risk reductions reported as high as about 41%.
- The results are observational and limited by wrist-level, 7-day measurement, a healthier UK Biobank sample, and 2014–2018 data collection, so authors call for replication and randomized or interventional trials before changing clinical practice; practical options to test include more daytime outdoor time and brighter indoor or workplace lighting.