Higher daytime light exposure was associated with a lower risk for incident dementia and outperformed several established dementia risk factors as a predictor of future dementia.

Results of a large prospective cohort study showed that daytime light exposure above 1000 lux was associated with a 16% lower risk for incident dementia compared with exposure below 1000 lux. Longer daily exposure to bright light also was associated with a further reduction in dementia risk.

The association was strongest among individuals with higher average nighttime light exposure, those with an evening chronotype, and APOE epsilon 4 carriers, the investigators reported.

“These findings position daytime light exposure as a novel, robust, and measurable indicator of dementia risk,” the investigators, led by Nana Zheng, Institute of Neuropsychiatry, The Affiliated Brain Hospital, Guangzhou Medical University, Guangzhou, China, wrote.

“They could inform future research into light-based interventions and public health guidelines that recommend increased daytime light exposure as a low-cost strategy to support brain health, particularly in high-risk populations,” the researchers added.

The findings were published online on June 24 in General Psychiatry.

Previous research has linked brighter daytime light exposure to improved circadian rhythms and circadian disruption to an increased risk for neurodegenerative diseases, including dementia. However, the direct association between daytime light exposure and dementia risk has remained unclear.

The goal of the study was to determine whether objectively measured daytime and nighttime light exposure predicted the risk for incident dementia.

The analysis included 87,577 dementia-free participants from the UK Biobank, a population-based cohort of more than 500,000 adults aged 40-73 years enrolled between 2006 and 2010. The mean participant age was 62.4 years, and 57.0% were women.

Participants wore an actigraphy device with a built-in light sensor that continuously measured indoor and outdoor daytime and nighttime light exposure over 7 days. Incident dementia was then assessed over a median follow-up of 8.1 years.

Researchers compared participants with average daytime light levels above vs below 1000 lux, equivalent to the brightness of an overcast day outdoors, and assessed the duration of brighter light exposure.

The primary outcome was incident dementia, identified using primary care, hospital admission, and death registry data. Analyses were adjusted for demographic, clinical, environmental, and lifestyle factors.

During a median follow-up of 8.1 years, 0.85% of participants developed incident dementia. Those participants were generally older and more likely to be men, have lower education, be less physically active, smoke, and have a history of diabetes, hypertension, or hearing loss than those who remained dementia-free.

Results showed that participants exposed to an average daytime light level above 1000 lux had a significantly lower risk for incident dementia than those exposed to lower light levels (adjusted hazard ratio [HR], 0.84; 95% CI, 0.71-0.99; P = .039).

Longer daytime exposure to bright light was associated with a lower risk for incident dementia across multiple intensity thresholds. At the 5000-lux threshold, participants exposed for more than 0.70 h/d had a 17% lower risk than those with shorter exposure (HR, 0.83; 95% CI, 0.70-0.99; P = .036).

Subgroup analyses showed that higher daytime light exposure was associated with a 30%-38% reduction in dementia risk among participants with higher nighttime light exposure, a 31%-41% reduction among those with an evening chronotype, and a 19%-27% reduction among APOE epsilon 4 carriers.

Viewing Dementia Risk in a New Light

The investigators said the findings suggest increasing daytime light exposure could be particularly beneficial in these high-risk groups.

“In other words, these findings suggest a targeted approach to mitigate dementia risk by increasing daytime light levels for these populations,” they wrote.

Researchers also ranked daytime light exposure against established predictors of dementia. Less than 0.70 hours of bright daytime light (≥ 5000 lux) had greater predictive value than several traditional risk factors, including alcohol consumption, obesity, air pollution, and hearing loss.

The investigators acknowledged several study limitations. The UK Biobank cohort is healthier than the general population, and the specific light exposure thresholds identified may not be directly applicable to populations with different medical profiles or lifestyles. In addition, light exposure was measured at the wrist, although light exerts its primary nonvisual health effects through ocular pathways.

The study was supported by Guangzhou Key Clinical Speciality (Clinical Medical Research Institute) and the Guangzhou Municipal Key Discipline in Medicine Project. The UK Biobank was supported by the Wellcome Trust, Medical Research Council, Department of Health, Scottish Government, Northwest Regional Development Agency, Welsh Government, British Heart Foundation, Cancer Research UK, and Diabetes UK. 

Disclosure information for study authors is available in the original study publication.