Light pollution caused by unprotected lights covers the city of Athens. Credit: C. Bouroussis
Excessive artificial light at night not only pollutes the sky, it can also pollute the brain, according to a surprising new study focusing on light pollution and Alzheimer's disease.
According to the Alzheimer’s Association, “Alzheimer’s is the most common cause of dementia, a general term for loss of memory and other cognitive abilities severe enough to interfere with daily life. Alzheimer’s disease accounts for 60 to 80 percent of dementia cases.” Although “not a normal part of aging,” those over age 65 make up the majority of cases. It typically begins with difficulty remembering new information and can progress to an inability to communicate or interact with the environment.
Now, a study led by Robin Voigt-Zuwala, an assistant professor at Rush University Medical Center in Chicago, implicates light pollution as a significant risk factor in the development of Alzheimer's. A study that was just published Frontiers in neuroscience strongly correlates exposure to ALAN with an increased incidence of Alzheimer's.
“A higher amount of outdoor night light was associated with a higher prevalence of Alzheimer’s disease,” Voigt-Zuwala and her team write. “While atrial fibrillation, diabetes, hyperlipidemia, hypertension, and stroke were more strongly associated with Alzheimer’s disease prevalence than night light intensity, night light was more strongly associated with Alzheimer’s disease prevalence than alcohol abuse, chronic kidney disease, depression, heart failure, and obesity.”
The researchers continue: “Surprisingly, exposure to night light is more strongly associated with the prevalence of Alzheimer’s disease in people younger than 65 years than any other disease factor examined.”
Given the complexity of Alzheimer's and related dementia, more studies will need to replicate the findings before light pollution can be pointed to as a factor, says Kallol Kumar Bhattacharyya, a researcher at Utah State University in Logan who was not involved in the work. In particular, he says, longitudinal studies (which follow subjects over long periods) and studies from other parts of the world will be needed. But “this is an excellent study with exciting findings,” he says.
Altered rhythms
In the new work, Voigt-Zuwala and her colleagues relied on publicly available data: maps of light pollution across the continental United States and government medical records pulled from sources such as Medicare. By combining the data sets, Voigt-Zuwala’s research team was able to categorize areas from the highest to lowest amount of ALAN.
According to her, the fact that people under 65 are more vulnerable to excess light could be explained by “certain genotypes (that)… influence the early onset of Alzheimer’s disease.” The disruption of circadian rhythms, which govern the body’s response to light signals, is probably one of the causes. According to her, “younger people are more likely to live in urban areas and have lifestyles that can increase light exposure at night.”
This exposure, by altering the body's most natural rhythms synchronized with light, can cause inflammation, including in the brain, which can promote an increase in beta-amyloid, a protein involved in Alzheimer's.
According to the National Institute on Aging, “A well-known gene that influences Alzheimer’s risk is the apolipoprotein E (APOE) gene. This gene is involved in making a protein that helps transport cholesterol and other types of fat in the bloodstream. Problems in this process are thought to contribute to the development of Alzheimer’s.”
Voigt-Zuwala’s study notes that “while it is difficult to speculate as to why individuals under 65 years of age would be particularly vulnerable to the effects of nighttime light exposure, the literature demonstrates that there are individual differences in light sensitivity. Indeed, APOE genotype, a factor influencing early onset of Alzheimer’s disease, affects response to biological stressors… and this could explain the increased vulnerability to the effects of nighttime light exposure…”
The study also notes that “at the biochemical level, exposing mice to dim light increases the production of pro-inflammatory chemicals.” For example, the mice have less “brain-derived neurotrophic factor (BDNF) in a brain region relevant to Alzheimer’s disease (i.e., the hippocampus)… This finding is intriguing since our group has shown that low levels of BDNF are a hallmark that precedes cognitive decline.” That is, low levels of BDNF can lead to increased brain inflammation.
A growing awareness
There are some caveats. First, the study did not account for exposure to indoor light pollution, but only looked at outdoor lighting at night. Second, the government health care data only captured where people were currently living, not their previous residences or places of birth.
Despite these factors, Voigt-Zuwala, a longtime researcher of circadian rhythms, was excited by the study's findings. “I felt like I had won,” she said. AstronomyHowever, he warns against “overinterpreting” the data.
The next step, Voigt-Zuwala said, is to secure funding for a much larger study that could go beyond the correlation between AD and ALAN and possibly establish a causal link. She has only just begun to “start to build this story.” A long-term study will take decades, starting with middle-aged participants and looking at both cognitive assessments and biological markers over time.
“The current study is certainly a step forward, but there is still a long way to go,” Battacharyya agrees.
However, the current study adds to the many harmful effects of light pollution, including, as the study notes, “sleep disturbance, obesity, depression, anxiety, memory dysfunction, atherosclerosis and cancer.”
Dark sky advocates can now also point to the possible link between excessive lighting and Alzheimer's as the astronomical community works to ally itself with environmental and public health advocates seeking a common cause: reducing light pollution in the name of science, nature and human health.
Related: How to win the battle against light pollution
As for anyone who is individually concerned about this newly discovered correlation between ALAN and Alzheimer's, the doctor has some practical advice: blackout curtains and eye masks. And those with a family history of early-onset Alzheimer's should definitely take these measures, advises Voigt-Zuwala.
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