Scientists have found a correlation between the intensity of nighttime outdoor lighting, which can disrupt sleep, and Alzheimer’s disease [1].
Light pollution – heavy consequences
Outdoor lights help us with a lot of things, such as illuminating our streets and highways, discouraging crime, and so on. City nightlines and views of Earth at night from the orbit are among the most iconic images of our civilization. However, bright lights during nighttime are also quite unnatural for our biology, which is built around circadian rhythms.
Excessive levels of artificial outdoor light are called “light pollution” and have been linked to sleep disruption, obesity, depression, anxiety, memory dysfunction, atherosclerosis, and cancer [2]. Indoor light was found to be a problem as well [3]. Laws exist in certain states and countries that limit light pollution levels.
Some of those conditions, especially sleep disruption and depression, are known comorbidities for Alzheimer’s disease. In a new study, researchers from Rush University Medical Center in Chicago set out to investigate a direct link between light pollution and Alzheimer’s.
One of the strongest factors
The scientists assigned a light pollution score to each US state and retrieved data on Alzheimer’s prevalence in those states, controlling for several known factors that contribute to Alzheimer’s: alcohol abuse, atrial fibrillation, chronic kidney disease, depression, diabetes, heart failure, hyperlipidemia, hypertension, obesity, and stroke.
In their analysis, average nighttime light intensity was significantly associated with Alzheimer’s prevalence. This correlation remained strong in both people under and over 65, males and females, and across all races except Asian Pacific islanders.
Average nighttime light intensity was associated with Alzheimer’s prevalence even when accounting for alcohol abuse, chronic kidney disease, depression, heart failure, and obesity. According to the researchers, this suggests that nighttime light intensity has a stronger influence on Alzheimer’s than any of these conditions.
However, other covariates had a stronger association with Alzheimer’s than light intensity. Those included atrial fibrillation, diabetes, hyperlipidemia, hypertension, and stroke, “indicating that nighttime light exposure had a more subtle effect than these disease covariates.” Interestingly, for people under the age of 65, average light intensity was associated with Alzheimer’s even when accounting for all the covariates.
There is, of course, a lot of heterogeneity inside states. Some areas are more populated and some less, which results in corresponding differences in light pollution. For a more fine-grained outlook, the researchers picked 45 counties from across the country and threw in the District of Columbia. Nighttime light intensity was determined for the counties that contain the largest city in each state, and this data was compared to Medicare Chronic Conditions’ county-level data on Alzheimer’s.
The correlation held strong on a county level, too. In fact, it was even stronger than on a state level:
Interestingly, a county’s population size did not seem to define its levels of light pollution:
Possible lifestyle changes
“We show that in the US there is a positive association between AD prevalence and exposure to light at night, particularly in those under the age of 65,” said the first author of the study, Dr. Robin Voigt-Zuwala, an associate professor at Rush University Medical Center. “Nightly light pollution, a modifiable environmental factor, may be an important risk factor for AD.”
Discussing the seemingly stronger correlation between light pollution and Alzheimer’s in the younger population, Voigt-Zuwala suggested that “younger people are more likely to live in urban areas and have lifestyles that may increase exposure to light at night.”
How can this risk factor be mitigated? “Awareness of the association should empower people, particularly those with risk factors for AD, to make easy lifestyle changes,” said Voigt-Zuwala. “Easy to implement changes include using blackout curtains or sleeping with eye masks. This is useful especially for those living in areas with high light pollution.”
Like any populational study, this one does not prove a causal relationship but only shows correlation. It also had several limitations; for instance, it didn’t account for people migrating during their lives. Still, the study highlights the importance of light pollution as part of the exposome, the set of environmental conditions that affect human health.
The analyses reveal that greater average nighttime light intensity (i.e., light pollution) was associated with higher AD prevalence. This was true for 2012–2018 average and each year examined individually, and in those over and under the age of 65 (i.e., 65+, <65), in both sexes, and in each race (except Asian Pacific Island which may be related to power). This finding was observed when examining data on the state level as well as on the county level.
Literature
[1] Voigt, R. M., Ouyang, B., & Keshavarzian, A. (2024). Outdoor nighttime light exposure (light pollution) is associated with Alzheimer’s disease. Frontiers in Neuroscience, 18, 1378498.
[2] Bożejko, M., Tarski, I., & Małodobra-Mazur, M. (2023). Outdoor artificial light at night and human health: a review of epidemiological studies. Environmental Research, 218, 115049.
[3] Sweeney, M. R., Nichols, H. B., Jones, R. R., Olshan, A. F., Keil, A. P., Engel, L. S., … & Jackson, C. L. (2024). Exposure to indoor light at night in relation to multiple dimensions of sleep health: findings from the Sister Study. Sleep, 47(2), zsad100.
