New research claims that exposure to outdoor light at night may increase the prevalence of Alzheimer’s disease, especially in people under the age of 65.

The researchers who conducted the study, funded by a National Institutes of Health grant and published in the journal Frontiers in Neuroscience on Friday, said they have found correlations 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 in place aimed at reducing light pollution, but the authors of the study say that despite this, the “levels of light at night remain high in many parts of the country”.

While “streetlights, roadway lighting, and illuminated signs can deter crime, make roads safer, and enhance landscaping”, undisrupted light, however, “comes with ecological, behavioral, and health consequences”, the authors said.

For this study, the researchers evaluated Alzheimer’s disease prevalence by looking at the average nighttime light intensity by state and county in the US from 2012 to 2018, using satellite-acquired light pollution data and Medicare data reports of Alzheimer’s prevalence. They also incorporated medical data about 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 associated more strongly with Alzheimer’s disease prevalence than nighttime light intensity, the authors said that nighttime light was more strongly associated with Alzheimer’s disease prevalence than “alcohol abuse, chronic kidney disease, depression, heart failure, and obesity”.

For people under 65, nighttime light exposure was more strongly associated with Alzheimer’s prevalence than any other disease factor examined in the study.

“This could suggest that younger people may be 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, which influence early-onset Alzheimer’s disease, impact the response to biological stressors which could account for increased vulnerability to the effects of nighttime light exposure,” adding that younger people are also more likely to live in “urban areas and have lifestyles that may increase exposure to light at night”.

Voigt-Zuwala said she believes that light is the “greatest factor” influencing circadian rhythms – our internal clock in our brains that signals when to be awake or asleep by responding to light changes in our environment.

Exposure to light at night disrupts those rhythms, which Voigt-Zuwala believes impacts Alzheimer’s disease. The group’s research, she said, has shown that disruption reduces resiliency, which, in essence “makes individuals more susceptible to disease”.

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

Living in an area with more intense outdoor light at night is associated with shorter sleep duration, increased daytime sleepiness and dissatisfaction with sleep quality, the study states.

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 quality brain health, to protect your brain over time from developing dementia, is having good quality sleep,” adding: “It wasn’t a surprise to see that evening light exposure that can fracture sleep is associated with dementia.”

But Karlawish did point out that the researchers themselves acknowledged some limitations and caveats to the study in their paper – including that the Medicare data is from current residences of individuals, and is not necessarily reflective of lifelong residences in these areas and exposure to light over time.

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

The authors also acknowledge in the paper that they evaluated the prevalence, not the incidence of Alzheimer’s, meaning that they measured the proportion of a population who have a specific characteristic at a given time rather than the rate of new cases of a disease occurring in a population over a period of time.

David Knopman, a clinical neurologist at the Mayo Clinic, expressed concerns about the methodology of the study and pointed out factors he believes ought to have been addressed.

Light exposure measured by satellites, he said, “doesn’t take into account window shades or natural sunlight exposure”, adding that the northern tier of the US has “more daylight hours in summertime and far fewer in the winter”.

Knopman mentioned that there were likely also health and socioeconomic status related differences at play not in the study; some rural areas have a lower amount of physicians per capita and therefore lower rates of dementia diagnoses, but not necessarily less dementia.

Voigt-Zuwala agreed that there were “many limitations associated with a population based study” and said that additional studies were needed to “understand how light inside the home impacted 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 it was the “impetus to conduct the county analysis” where she “compared counties with approximately the same population and population density to address this exact point”.

“The assumption is that in urban centers of approximately the same population and density that access to physicians, exposure to air pollution, and other factors that could be important,” she said. “However, this approach does not give us individual level data which is why more studies are needed in the future.”

“There is much left to learn on this topic and I’m looking forward to spearheading these efforts,” she added.

While the data from preclinical studies and this study suggest that exposure to light at night may influence Alzheimer’s disease, the study notes that additional studies evaluating clinical and population health are needed, including those looking into the impacts of the “the totality of outdoor and indoor nighttime light exposure”.

The authors say they hope the research empowers people to “make easy lifestyle changes” such as using blackout curtains or sleeping with eye masks.