Dementia is one of the leading causes of disability and dependence in older adults, yet reliable and modifiable predictors of dementia risk remain limited. A new study published in General Psychiatry investigated whether objectively measured daytime and nighttime light exposure is associated with future dementia risk. Using wearable-device data from more than 87,000 dementia-free participants in the UK Biobank, the researchers examined how patterns of daily light exposure relate to incident dementia over a median follow-up of 8.1 years, findings of which suggest that greater exposure to bright daytime light may be an important, potentially modifiable factor in promoting long-term brain health.
Using seven-day wrist-worn light-sensor measurements, the study assessed average daytime and nighttime light exposure, as well as the duration of bright-light exposure at different intensity thresholds. During follow-up, 741 participants developed dementia. Individuals exposed to daytime light levels above 1,000 lux had a 16% lower risk of dementia compared with those exposed to lower light levels. Longer durations of bright light exposure, such as at least 0.70 hours per day at 5,000 lux or higher, were also associated with a significantly reduced risk of developing dementia. In contrast, nighttime light exposure was not significantly associated with dementia risk.
The researchers further explored potential biological mechanisms linking daytime light exposure to cognitive health. Mediation analyses suggested that circadian rest–activity rhythms and structural brain measures could explain up to one-third of the observed association, with the fusiform cortex emerging as a significant mediator. The protective effects of daytime light exposure were particularly pronounced among individuals with high nighttime light exposure, an evening chronotype, or the presence of the APOE ε4 genetic risk variant, with dementia risk reductions ranging from 19% to 41%.
“In our study, high daytime light levels were associated with increased surface area or volume in the medial temporal lobe, possibly contributing to the observed interaction effects.” the authors noted. Notably, low exposure to bright daytime light (less than 0.70 hours per day at ≥5,000 lux) ranked as a stronger predictor of dementia than several traditional risk factors, including alcohol consumption, obesity, hearing loss and traumatic brain injury. The study design and key findings are summarized in the figure below.
The authors caution that the study was observational and cannot establish causality. In addition, light exposure was measured at the wrist rather than directly at the eye, and the UK Biobank population is generally healthier than the wider population, which may limit generalizability. Nevertheless, the results highlight daytime light exposure as a simple, low-cost and objectively measurable factor that could contribute to dementia prevention strategies. Future studies are needed to determine whether increasing daytime light exposure through environmental modifications or light-based interventions can directly reduce dementia risk.
General Psychiatry
Observational study
Associations between wearable-device-measured daytime and nighttime light exposures and dementia risk: A prospective cohort study
24-Jun-2026