A new study suggests that exposure to outdoor light at night may increase the risk of Alzheimer’s disease, especially in people under the age of 65.
Researchers who conducted the study, which was funded by a National Institutes of Health grant and published Friday in the journal Frontiers in Neuroscience, said they found a correlation between areas of the U.S. that experience excessive exposure to artificial light at night and the prevalence of Alzheimer’s disease.
At least 19 US states have laws in place to reduce light pollution, but the study authors say that despite this, “night-time light levels remain high in many parts of the country.”
“Street lighting, roadway lighting, and illuminated signs can deter crime, make roads safer, and enhance aesthetics,” but uninterrupted light is “associated with ecological, behavioral, and health effects,” the authors say.
In the study, the researchers used satellite-derived light pollution data and Medicare data reports on Alzheimer’s disease prevalence to assess the prevalence of Alzheimer’s disease by average nighttime light intensity by U.S. state and county from 2012 to 2018. They also incorporated medical data on variables that are known or thought to be risk factors for Alzheimer’s disease into their analysis.
While diseases such as diabetes and hypertension were more strongly associated with Alzheimer’s prevalence than night-time light intensity, night-time light was more strongly associated with Alzheimer’s prevalence than “alcohol abuse, chronic kidney disease, depression, heart failure, and obesity,” the authors said.
For people under 65, exposure to light at night was more strongly associated with the prevalence of Alzheimer’s disease than any other disease factor examined in the study.
“This may suggest that young people may be particularly sensitive to the effects of night-time light exposure,” the researchers said.
“Specific genotypes that influence early-onset Alzheimer’s disease may influence responses to biological stressors and contribute to increased vulnerability to the effects of night-time light exposure,” explained Dr. Robin Voigt-Zwala, an associate professor at Rush University Medical Center and one of the study’s authors, adding that younger people are more likely to “live in urban areas and lead lifestyles that may increase their exposure to light at night.”
Voigt-Zwala said she believed light was the “biggest factor” affecting our circadian rhythm – the internal clock in our brain that responds to changes in light in the environment and tells us when to wake up or go to sleep.
Voigt-Zwala believes that exposure to light at night disrupts these rhythms, which in turn contributes to Alzheimer’s, because her group’s research has shown that disrupted rhythms reduce resilience, essentially “making you more susceptible to disease,” she said.
Samuel Gandy, an Alzheimer’s disease researcher at the Mount Sinai Institute in New York, said the study’s overall findings make sense because “light controls circadian rhythms, which control sleep,” adding that in the case of Alzheimer’s, “poor quality sleep increases risk.”
Living in areas with strong outdoor light at night can lead to shorter sleep duration, increased daytime sleepiness and dissatisfaction with sleep quality, the study says.
“One of the pillars of good brain health that protects the brain against the development of dementia in the long term is getting good quality sleep,” said Dr. Jason Karlawish, co-director of the Penn Memory Center at the University of Pennsylvania, who was not involved in the study. “It’s not surprising that exposure to evening light, which disrupts sleep, is associated with dementia,” he added.
But Karlawish noted that the researchers themselves acknowledged in their paper that the study has several limitations and caveats, including that the Medicare data is for individuals’ current locations of residence and does not necessarily reflect lifetime residence in those areas or long-term light exposure.
Still, Karlawish said the study “reaffirms the importance of one of the pillars of brain health.”
The authors also acknowledge in their paper that they assessed the prevalence, rather than incidence, of Alzheimer’s disease — that is, they measured the proportion of the population with a particular characteristic at a particular point in time, rather than the rate at which the disease newly occurs in a population over a period of time.
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David Knopman, a clinical neurologist at the Mayo Clinic, expressed concerns about the study’s methodology, pointing out factors he believes should have been addressed.
He said satellite-measured light exposure “doesn’t take into account window blinds or natural light exposure,” adding that in the northern U.S., “the hours of daylight are longer in the summer and much shorter in the winter.”
Knopman noted that differences related to health status and socioeconomic status, which did not influence the study, also likely played a role: Some rural areas have fewer doctors per capita and therefore lower dementia diagnoses, but that doesn’t necessarily mean they have less dementia.
Voigt-Zwala agreed that “population-based studies have many limitations” and said additional research is needed “to understand how light levels in the home influenced these results.” Differences in daylight hours across the country will “absolutely” be taken into account in future research.
“Rural and urban areas have different levels of light pollution,” she said, adding that this “triggered us to do a county analysis” comparing “counties with similar populations and population densities to really address this.”
“The assumption is that in urban centers with similar population and population density, access to physicians, exposure to air pollution and other factors may be important,” she said, “but this approach doesn’t provide individual-level data, so more research is needed.”
“There is still much to learn on this topic, and I look forward to spearheading these efforts,” she added.
While preclinical studies and the data from this study suggest that exposure to light at night may play a role in Alzheimer’s disease, the study notes that additional research evaluating clinical and population health is needed, including studies examining the effects of “the totality of outdoor and indoor nighttime light exposure.”
The authors say they hope the study will encourage people to make “simple lifestyle changes” such as using blackout curtains or sleeping with an eye mask.