Time spent in daylight could lower the risk of dementia and offer extra benefits for people at particularly high risk, new research suggests. The findings, published in General Psychiatry, could provide a low-cost way to support brain health.
Having less than 0.7 hours of bright daytime light per day was a stronger predictor of dementia than six traditional risk factors. Moderately bright natural light exposure—equivalent to an overcast day outdoors—was linked with a 16% reduction in the risk of dementia.
“Daytime light exposure may serve as a novel indicator of dementia risk,” said researcher Hongliang Feng, PhD, from Guangzhou Medical University in China. Natural cycles of darkness at night followed by bright light during the day are fundamental to entrain circadian rhythms. These regulate physiology, behavior, and cognition, with circadian disruptions common among people with dementia.
Noting that modern lifestyles limit daytime exposure to natural light, the researchers investigated exposure to day- and nighttime light using actigraphy devices that track body movements with built-in light sensors.
The study included 87,577 UK Biobank participants who wore accelerometers on their wrists to measure physical activity and natural light exposure for seven days. Over a median follow-up of 8.1 years, 741 of these people (0.85%) developed dementia.
Higher daytime light, both in terms of average exposure and the duration in bright light, was significantly associated with a lower dementia risk. Daytime light exposure above 1000 lux—a moderately bright light level equal to an overcast day outdoors—was associated with a hazard ratio of 0.84 for dementia. Longer exposure to brighter light of at least 0.70 hours with at least 5000 lux was linked with a further risk reduction, and a hazard ratio of 0.83.
In exploratory analyses, circadian rest-activity rhythms (CRAC) and brain structures mediated up to a third of this association, supporting the idea that improvements in circadian rhythms may have contributed to these results. Dementia protection from light exposure was stronger in people with high levels of nighttime light exposure, those with a “night owl” evening chronotype, or who carried the APOE ε4 allele, with a risk reduction of up to 41%.
Having more than 0.7 hours per day of bright daytime light of at least 5000 lux outperformed the established dementia risk predictors, including alcohol consumption, obesity, air pollution, hearing loss, use of vitamin D supplements, and traumatic brain injury.
However, nighttime light showed no significant association with dementia risk.
The findings point to the importance of higher daytime light exposure in reducing the chances of dementia and offer a simple, cost-free way to reduce this risk.
“Practical implementation pathways could include optimizing indoor lighting at home, community‐based outdoor activity promotion programs, and workplace lighting modifications designed to increase daytime light exposure, such as ensuring adequate illumination and access to natural light,” the researchers suggested.
They added: “Our findings underscore a more pronounced protective association of daytime light exposure in individuals with higher average nighttime light exposure, an evening chronotype, or APOE ε4 carrier status.
“In other words, these findings suggest a targeted approach to mitigate dementia risk by increasing daytime light levels for these populations.”
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