Guangzhou study of 88,000 older adults links 42 minutes of bright light daily to 17% lower dementia risk

Researchers led by Guangzhou Medical University in China analyzed data on nearly 88,000 older adults, sciencealert.com reports. Those with more daily light exposure were less likely to develop dementia over the following decade or so. sciencealert.comsciencealert.comsciencealert.comOpen source ↗ Spending at least 42 minutes a day in bright light of 5,000 lux or more was associated with a 17 percent lower relative risk of dementia. The results were published in General Psychiatry.

Study design and results

The data came from the UK Biobank, where participants wore a light sensor continuously for seven days. Those exposed to more daytime light during that week had a significantly lower risk of dementia roughly eight years later. The longer the exposure to natural or artificial light, the stronger the association.

According to the authors, daytime light exposure was a better predictor of dementia than six traditional risk factors, including obesity, alcohol consumption, air pollution and hearing loss. The association was stronger in "night owls": greater daytime light exposure was linked with up to a 41 percent lower risk. It was also particularly evident in people with higher nighttime light exposure and in carriers of genetic risk factors. Among carriers of the APOE ε4 allele, more daytime light was associated with up to a 27 percent lower risk.

Possible mechanisms and limits

The study is observational, and it is not known how light exposure might affect cognitive decline. Participants with more daytime light tended to have greater brain volume or surface area in parts of the medial temporal lobe, a region important for memory that is often among the first affected by dementia. sciencealert.comsciencealert.comsciencealert.comOpen source ↗

The authors also consider indirect routes. Earlier studies found that daytime light levels predict a lower risk of major depressive disorder, which is a risk factor for dementia. Light entering the eyes activates the brain's master clock, which regulates the body's daily rhythms, and sunlight on the skin stimulates vitamin D production, which is also tied to dementia risk.

The authors point out that people in modern society spend nearly 90 percent of the day indoors under artificial lighting, which typically provides about 300–500 lux during the day. They suggest that brighter daytime light (for example, 1000 lux or more) may be a simple means of preventing psychiatric disorders and dementia. They say the findings could inform research into light-based interventions, and note that no earlier study had examined average daytime and nighttime light exposure in relation to dementia risk.