Light pollution at night may increase the risk of Alzheimer’s, study finds

New research shows that exposure to outdoor light in the evening may increase the risk of Alzheimer’s disease, especially in people under 65.

The researchers who conducted the study, funded by a grant from the National Institutes of Health and published in the journal Frontiers in neuroscience said Friday that they have found links between areas of the US with excessive exposure to artificial light at night and the prevalence of Alzheimer’s disease.

In the US, at least 19 states have legislation aimed at reduce light pollutionbut the study authors say that despite this, “light levels at night remain high in many parts of the country”.

While “street lighting, roadway lighting, and illuminated signs can deter crime, make roads safer, and beautify landscapes,” undisturbed light has “ecological, behavioral, and health consequences,” the authors say.

For this study, researchers evaluated the prevalence of Alzheimer’s disease by looking at average nighttime light intensity by state and county in the US from 2012 to 2018, using satellite obtained light pollution data and Medicare data reports on Alzheimer’s prevalence. They also incorporated medical records on variables known or believed to be risk factors for Alzheimer’s disease into their analysis, they said.

While conditions such as diabetes, hypertension and others were more strongly associated with the prevalence of Alzheimer’s disease than nighttime light intensity, the authors said nighttime light was more strongly associated with the prevalence of Alzheimer’s disease than “alcohol abuse, chronic kidney disease, depression, heart failure and obesity.”

In people younger than 65, nighttime light exposure was found to have a stronger association with Alzheimer’s prevalence than any other disease factor examined in the study.

“This could mean that younger people are particularly sensitive to the effects of light exposure at night,” the researchers said.

Dr. Robin Voigt-Zuwala, an associate professor at Rush University Medical Center and one of the authors of the research paper, explained: “Certain genotypes that influence the early onset of Alzheimer’s disease affect the response to biological stressors, which may lead to greater vulnerability to the effects of light exposure at night.” He added that younger people are also more likely to live in “urban areas and have lifestyles that may increase light exposure at night.”

Voigt-Zuwala said she believes light is the “biggest factor” influencing circadian rhythms – our internal clock in our brain that tells us when we should be awake or asleep by responding to changes in the light in our environment.

Exposure to light at night disrupts those rhythms, which Voigt-Zuwala believes influences Alzheimer’s. The group’s research, she said, has shown that disruption reduces resilience, essentially “making people more susceptible to disease.”

Samuel Gandy, an Alzheimer’s researcher at Mount Sinai in New York, said the overall findings of the new study make sense because “light controls circadian rhythm and that controls sleep,” he said, adding that for Alzheimer’s, “poor sleep increases the risk.”

The study found that living in an area with more intense outdoor light at night was associated with shorter sleep duration, more daytime sleepiness, and dissatisfaction with sleep quality.

Dr. Jason Karlawish, co-director of the Penn Memory Center at the University of Pennsylvania, who was not involved in the study, said, “One of the pillars of good brain health, to protect your brain from developing dementia over time, is a good night’s sleep,” adding, “It was no surprise that evening light exposure, which can disrupt sleep, is linked to dementia.”

But Karlawish did point out that the researchers themselves acknowledge in their paper some limitations and caveats to the study, including that the Medicare data come from individuals’ current areas of residence, and do not necessarily reflect lifetime residence in those areas and light exposure over time.

Still, Karlawish said the study “reiterates the importance of one of the pillars of brain health.”

The authors also acknowledge in the article that they evaluated the prevalence, not the incidence, of Alzheimer’s. That is, they measured the percentage of a population that has a specific characteristic at a given time, rather than the number of new cases of a disease that occur in a population over a given period.

David Knopman, a clinical neurologist at the Mayo Clinic, raised concerns about the study’s methodology and pointed to factors that he said should have been addressed.

He said the light exposure measured by satellites does not account for sun protection or exposure to natural sunlight, adding that the northern US “has more daylight hours in the summer and much less in the winter.”

Knopman noted that there were likely also differences in health and socioeconomic status that were not captured in the study. In some rural areas, there are fewer physicians per capita and therefore fewer dementia diagnoses, but that does not necessarily mean there is less dementia.

Voigt-Zuwala agreed that there are “many limitations to a population-based study” and said additional studies were needed to “understand how light in the home affects these outcomes.” The difference in daylight hours across the country is something she would “absolutely” take into account in future studies.

“Rural and urban areas have different levels of light pollution,” she said, adding that this was the “impetus to do the county analysis” comparing “counties with approximately the same population and population density to address this exact issue.”

“The assumption is that in urban centers with roughly the same population and density, access to doctors, exposure to air pollution and other factors that may be important,” she said. “However, this approach does not give us individual-level data, so more studies are needed in the future.”

“There is still much to learn on this topic and I look forward to spearheading these efforts,” she added.

While data from preclinical studies and this study suggest that nighttime light exposure may impact Alzheimer’s disease, the study notes that additional studies are needed that evaluate clinical and public health outcomes, including studies that look at the effects of “the totality of indoor and outdoor nighttime light exposure.”

The authors hope the study will enable people to “make simple lifestyle changes,” such as using blackout curtains or sleeping with sleep masks.