Brighter daytime light is linked to a lower dementia risk

Circadian rhythms may influence dementia development

Dementia is the most common neurodegenerative disease worldwide and is rapidly increasing in prevalence with the aging of the global population. With limited effective treatments, prevention is a top priority.

The environmental light-dark cycle is key to aligning with natural circadian rhythms, which, in turn, are fundamental to normal physiological, behavioral, and cognitive functioning. People with dementia often show disrupted circadian rhythms, and conversely, these are linked to a higher risk of dementia.

More evidence of such associations comes from the promising use of bright light therapy (BLT) in patients with dementia to correct dysregulated circadian rhythms and improve cognitive symptoms. However, the modern lifestyle limits adequate exposure to natural bright light during the day, as most people spend this time indoors in buildings with insufficient lighting. This is worsened by excessive nighttime light exposure, which affects nearly 80% of the world’s population.

The current study aimed to investigate associations between daytime and nighttime light exposures and dementia risk.

Wearable devices tracked weekly light exposure patterns

Researchers measured daytime and nighttime light exposure using wrist-worn devices over seven days in a community cohort of 87,577 dementia-free adults (mean age 62 years). They then examined whether light exposure predicted future dementia diagnoses.

Brighter daytime light linked to lower dementia risk

Over a median follow-up period of eight years, 741 participants developed dementia. At baseline, participants who developed dementia were more likely to be older, male, less educated, less physically active, smokers, or to have hypertension, diabetes, or hearing loss.

The investigators found that exposure to daytime light levels brighter than 1,000 lux was associated with a 16% lower risk of dementia. Shorter durations of increasingly bright daytime light showed similar associations, suggesting a graded pattern across brighter-light thresholds.

Thus, 17% reductions in risk were observed with at least 1.4 hours of exposure to bright daytime light of 3000 lux or more, versus 0.7 hours at 5000 lux, and 0.45 hours at 7000 lux.

Greater effects among high-risk groups

The effects of higher average daytime light exposure and longer exposure to bright daytime light were most pronounced among certain groups. For instance, such exposures were associated with a 30% to 38% reduction in dementia risk among individuals exposed to higher nighttime light levels.

The corresponding reductions in those with an evening chronotype were 31%-41% and 19%-27% among carriers of the APOE ε4 gene variant, a major risk factor for Alzheimer's disease. However, no significant association was found between nighttime light exposure and dementia risk.

These four measures of daytime light exposure were compared with 15 established risk markers for dementia to assess their predictive ability. The highest-ranked daytime light measure was less than 0.7 hours of exposure to 5000 lux or more, ranking 10th among 19 predictors overall.

This daytime light measure outperformed six conventional factors: alcohol consumption, obesity, fine particulate matter air pollution, vitamin D supplement use, traumatic brain injury, and hearing loss. The last three also performed worse than exposures of less than 1.4 h at 3000 lux and 0.45 h at 7000 lux.

Potential mechanisms

The researchers explored possible mediators for these associations. The findings suggest that the fusiform cortex accounted for about 9% of the association between average daytime light level >1000 lux and dementia risk.

Exploratory analyses suggested that up to 33% of the association might be explained by changes in the circadian rest-activity rhythms and alterations in brain structures, although many of these mediation findings were exploratory and did not remain statistically significant after correction for multiple testing.

The findings also suggest that circadian rest-activity rhythms and structural brain changes may partly mediate the association between greater daytime light exposure and lower dementia risk. Exploratory analyses also suggested that greater daytime light exposure was associated with protective structural changes in the occipital, frontal, and temporal cortices, consistent with earlier studies, and suggest another protective mechanism, although these findings require validation in future studies.

Implications

The study identifies higher daytime light exposure as a significant correlate of lower dementia risk, with better predictive performance compared to several traditional risk markers. The findings are consistent with previous studies suggesting improvement in circadian rhythms and mental health, including major depression, with bright light exposure in the daytime.

Free‐living light exposure can also be easily measured objectively, potentially providing a promising method to identify individuals at high risk of dementia. Second, increasing daytime light exposure may be a simple and cost‐free strategy to reduce dementia risk in both clinical and community settings.

They suggest implementing simple steps, such as optimizing home and workplace lighting conditions and promoting engagement in community outdoor activities, targeting high-risk groups for these programs.

Why not nighttime lighting?

The lack of correlation with nighttime light exposure could be due to the use of wrist monitors, which may miss true retinal light exposure and do not capture blue light emission, which is characteristic of most light-emitting diodes and is the most powerful disruptor of circadian rhythms. Moreover, the study period predated the intense, frequent nighttime light exposure associated with the current widespread use of personal electronic devices at night.

Limitations

The study was observational and cannot prove that greater light exposure caused the lower dementia risk. This cohort was part of the UK Biobank study and is healthier and less deprived than the general population, which limits generalizability. The exact thresholds of light exposure used here may not be directly applicable across populations.

Weekly light exposure may not be consistent with long-term behaviors. The light-tracking devices could not capture retinal light perception, which is crucial to dementia risk associations.

Brighter days were linked to healthier brain aging

Higher daytime light exposure was associated with a significantly lower risk of dementia in older adults, whereas nighttime light exposure showed no significant association. Further research is needed to determine whether increasing daytime light exposure can directly reduce dementia risk and whether light-based interventions could potentially become part of dementia prevention strategies.

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