Nighttime Outdoor Light Pollution Linked to Alzheimer's Risk

Oct 17, 2024

Patrice Wendling

September 06, 2024

Living in areas saturated with artificial outdoor light at night is associated with an increased risk for Alzheimer's disease (AD), a new national study suggested.

Analyses of state and county light pollution data and Medicare claims showed that areas with higher average nighttime light intensity had a greater prevalence of AD.

Among people aged 65 years or older, AD prevalence was more strongly associated with nightly light pollution exposure than with alcohol misuse, chronic kidney disease, depression, or obesity.

In those younger than 65 years, greater nighttime light intensity had a stronger association with AD prevalence than any other risk factor included in the study.

"The results are pretty striking when you do these comparisons and it's true for people of all ages," Robin Voigt-Zuwala, PhD, lead author and director, Circadian Rhythm Research Laboratory, Rush University, Chicago, told Medscape Medical News.

The study was published online on September 6 in Frontiers of Neuroscience.

Exposure to artificial outdoor light at night has been associated with adverse health effects such as sleep disruption, obesity, atherosclerosis, and cancer, but this is the first study to look specifically at AD, investigators noted.

Two recent studies reported higher risks for mild cognitive impairment among Chinese veterans and late-onset dementia among Italian residents living in areas with brighter outdoor light at night.

For this study, Voigt-Zuwala and colleagues examined the relationship between AD prevalence and average nighttime light intensity in the lower 48 states using data from Medicare Part A and B, the Centers for Disease Control and Prevention, and NASA satellite–acquired radiance data.

The data were averaged for years 2012-2018 and states divided into five groups based on average nighttime light intensity.

The darkest states were Montana, Wyoming, South Dakota, Idaho, Maine, New Mexico, Vermont, Oregon, Utah, and Nevada. The brightest states were Indiana, Illinois, Florida, Ohio, Massachusetts, Connecticut, Maryland, Delaware, Rhode Island, and New Jersey.

Analysis of variance revealed a significant difference in AD prevalence between state groups (P < .0001). Multiple comparisons testing also showed that states with the lowest average nighttime light had significantly different AD prevalence than those with higher intensity.

The same positive relationship was observed when each year was assessed individually and at the county level, using data from 45 counties and the District of Columbia.

The investigators also found that state average nighttime light intensity is significantly associated with AD prevalence (P = .006). This effect was seen across all ages, sexes, and races except Asian Pacific Island, the latter possibly related to statistical power, the authors said.

When known or proposed risk factors for AD were added to the model, atrial fibrillation, diabetes, hyperlipidemia, hypertension, and stroke had a stronger association with AD than average nighttime light intensity.

Nighttime light intensity, however, was more strongly associated with AD prevalence than alcohol abuse, chronic kidney disease, depression, heart failure, and obesity.

Moreover, in people younger than 65 years, nighttime light pollution had a stronger association with AD prevalence than all other risk factors (P = .007).

The mechanism behind this increased vulnerability is unclear, but there may be an interplay between genetic susceptibility of an individual and how they respond to light, Voigt-Zuwala suggested.

"APOE4 is the genotype most highly associated with Alzheimer's disease risk, and maybe the people who have that genotype are just more sensitive to the effects of light exposure at night, more sensitive to circadian rhythm disruption," she said.

The authors noted that additional research is needed but suggested light pollution may also influence AD through sleep disruption, which can promote inflammation, activate microglia and astrocytes, and negatively alter the clearance of amyloid beta, and by decreasing the levels of brain-derived neurotrophic factor.

"It's a good article and it's got a good message, but I have some caveats to that," said George C. Brainard, PhD, director, Light Research Program, Thomas Jefferson University in Philadelphia, and a pioneer in the study of how light affects biology including breast cancer in night shift workers.

The biggest caveat, and one acknowledged by the authors, is that the study didn't measure indoor light exposure and relied instead on satellite imaging.

"They're very striking images, but they may not be particularly relevant. And here's why: People don't live outdoors all night," Brainard told Medscape Medical News.

Instead, people spend much of their time at night indoors where they're exposed to lighting in the home and from smartphones, laptops, and television screens.

"It doesn't invalidate their work. It's an important advancement, an important observation," Brainard said. "But the important thing really is to find out what is the population exposed to that triggers this response, and it's probably indoor lighting related to the amount and physical characteristics of indoor lighting. It doesn't mean outdoor lighting can't play a role. It certainly can."

Reached for comment, Erik Musiek, MD, PhD, a professor of neurology whose lab at Washington University School of Medicine in St. Louis has extensively studied circadian clock disruption and AD pathology in the brain, said the study provides a 10,000-foot view of the issue.

For example, the study was not designed to detect whether people living in high light pollution areas are actually experiencing more outdoor light at night and if risk factors such as air pollution and low socioeconomic status may correlate with these areas.

"Most of what we worry about is do people have lights on in the house, do they have their TV, their screens up to their face late at night, and this can't tell us about that," Musiek told Medscape Medical News. "But on the other hand, this kind of light exposure is something that public policy can affect."

"It's hard to control people's personal habits nor should we probably, but we can control what types of bulbs you put into streetlights, how bright they are, and where you put lighting in a public place," he added. "So I do think there's value there."

At least 19 states, the District of Columbia, and Puerto Rico have laws in place to reduce light pollution, with the majority doing so to promote energy conservation, public safety, aesthetic interests, or astronomical research, according to the National Conference of State Legislatures.

To respond to some of the limitations in this study, Voigt-Zuwala is writing a grant application for a new project to look at both indoor and outdoor light exposure on an individual level.

"This is what I've been wanting to study for a long time, and this study is just sort of the stepping stone, the proof of concept that this is something we need to be investigating," she said.

Voigt-Zuwala reported RO1 and R24 grants from the National Institutes of Health (NIH), co-author Ali Keshavarzian reported an NIH R24 grant, and co-author Bichun Ouyang reported having no conflicts of interest. Brainard reported having no relevant conflicts of interest. Musiek reported research funding from Eisai Pharmaceuticals.

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