Study details

For the study, researchers evaluated 87,577 dementia-free participants in the U.K. Biobank cohort who had daytime and nighttime free-living light exposures that were measured by wrist-worn accelerometry for a week. Dementia diagnoses were assessed by primary care records, hospital inpatient admissions, and death registry data.

Over eight years of follow up, the researchers found that people who had an average daytime light exposure above 1,000 lux, which is roughly the amount of light on an overcast day outside, had a 16% reduced dementia risk.

Exposure to bright daytime light of at least 5,000 lux for at least 42 minutes per day was also associated with a lower dementia risk, the researchers found.

Nighttime light had no significant association with dementia risk. The researchers noted that in later analyses, circadian rest-activity rhythms and specific brain structures mediated up to 33% of the dementia risk.

The researchers noted the study had several limitations. Participants in the U.K. Biobank cohort are typically healthier and less socioeconomically deprived than other populations. While seven-day monitoring can capture weekly trends, it may not capture long-term patterns. Light was also measured in the study at the wrist rather than eye level, the researchers noted.

They added that light exposure data was collected between 2014 and 2018, before LED lighting and increased nighttime device use was more widespread.

As for why light seemed to reduce dementia risk, Feng said "the core pathway is circadian regulation: bright daytime light stabilizes rest-activity rhythms and preserves key brain structures such as the fusiform cortex."

"Preclinical evidence also indicates daytime bright light may reduce neuroinflammation and slow amyloid-beta aggregation," he added. "Notably, vitamin D levels did not mediate the effect, meaning the cognitive benefit comes from direct neural and circadian effects, not sun-induced vitamin D production."

Dayan Goodenowe, a neuroscientist who was not involved in the study, said he believes the study "reinforces the idea that Alzheimer's disease is influenced by multiple modifiable factors and healthy brain function depends on many interconnected systems … [including] circadian regulation, sleep, metabolism, membrane biology, and nutrition."

For clinicians, Feng suggested that the study's findings "support recommending regular daytime bright light exposure as a simple, zero-cost, low-risk measure to support long-term cognitive health, particularly for patients at elevated dementia risk."