Higher Daytime Light Exposure Linked to Lower Dementia Risk

People exposed to higher levels of daytime light had a lower risk of dementia, prospective data from 88,000 U.K. Biobank participants showed.

Over 8 years of follow-up, people who had an average daytime light exposure above 1,000 lux — a moderately bright level of light, equivalent to an overcast day outdoors — had a 16% reduced dementia risk (HR 0.84, 95% CI 0.71-0.99, P=0.039), reported Hongliang Feng, MD, PhD, of Guangzhou Medical University in China, and co-authors.

Exposure to bright daytime light of at least 5,000 lux for at least 0.70 hr/day (42 minutes) also was tied to lower dementia risk (HR 0.83, 95% CI 0.70-0.99, P=0.036), Feng and colleagues reported in General Psychiatry.

“This exposure metric outperformed six traditional dementia risk factors, including obesity, alcohol intake, and traumatic brain injury, in predictive strength,” Feng said. “The protective effect was most pronounced in high-risk groups — evening chronotypes, people with high nighttime light exposure, and APOE4 carriers — with risk reduction reaching up to 41%,” he told MedPage Today.

Nighttime light had no significant association with dementia risk. In exploratory analyses, circadian rest-activity rhythms and specific brain structures mediated up to 33% of the dementia risk, the researchers noted. Vitamin D was not a mediator.

“The core pathway is circadian regulation: bright daytime light stabilizes rest-activity rhythms and preserves key brain structures such as the fusiform cortex,” Feng said.

“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.”

Light exposure is a primary cue for the circadian system. A number of studies have linked Alzheimer’s disease and dementia risk with circadian disruption or poor sleep, and some have tied poor sleep in midlife with faster brain aging.

“While bright light therapy is known to improve symptoms in patients with established dementia, this is the largest prospective cohort to date using objective wearable measurements to link routine, real-world daytime light exposure to reduced dementia risk in the general population,” Feng pointed out.

Feng and colleagues evaluated 87,577 dementia-free participants in the U.K. Biobank cohort who had daytime and nighttime free-living light exposures measured by wrist-worn accelerometry for 7 days. Incident dementia was ascertained by primary care records, hospital inpatient admissions, and death registry data.

The mean age of the study group was 62 and 57% were women. Light exposure fell into two distinct periods: 7:30 to 20:30, and 0:30 to 6:00. Over a median follow-up of 8.1 years, 741 participants developed dementia.

The study had several limitations, the researchers noted. U.K. Biobank participants are typically healthier and less deprived socioeconomically than other populations, they acknowledged. While 7-day monitoring can capture weekly trends, it might not reflect long-term patterns. In addition, light exposure was measured at the wrist, not at eye level.

Light exposure data were collected between 2014 and 2018, before the widespread adoption of LED lighting and increased nighttime device use, the researchers added.

“For frontline clinicians, these 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,” Feng suggested.