A street lamp outside your window, the orange charging light on a plug adapter next to your nightstand, a flash of your phone late at night indicating you’ve received an email—these artificial light sources would never occur in nature, and recent studies suggest they harm your brain.
Higher levels of artificial light at night were linked to increased stress-related activity in the brain, inflamed arteries, and a higher risk of heart disease, according to a preliminary study presented at the American Heart Association’s Scientific Sessions 2025.
Artificial light at night, or nighttime light pollution, is a nearly universal feature of modern cities, according to the study. This first-of-its-kind study reviewed brain scans and satellite images to show a biological pathway linking nighttime light exposure to heart disease.
“We know that environmental factors, such as air and noise pollution, can lead to heart disease by affecting our nerves and blood vessels through stress. Light pollution is very common; however, we don’t know much about how it affects the heart,” said study senior author Shady Abohashem, head of cardiac PET/CT imaging trials at Massachusetts General Hospital and an instructor at Harvard Medical School in Boston.
450 adult patients underwent identical PET/CT scans, and all of whom were without heart disease and no active cancer.
“This is a routine imaging test at my hospital,” Abohashem said. “The CT portion provides detailed anatomy, while the PET portion reveals metabolic activity in tissues. Using both imaging techniques together allows for the measurement of brain stress activity and arterial inflammation in a single scan.”
The analysis found that people exposed to higher levels of artificial light at night had higher brain stress activity, blood vessel inflammation and a higher risk of major heart events.
This information was collected from medical records and evaluated by two cardiologists who were blinded, meaning they were unaware of any information that could influence their decisions.
The greater the artificial night light exposure, the higher the risk of heart disease development. Every standard deviation increase in light exposure was associated with about 35% and 22% increased risk of heart disease over five- and 10-year follow-up periods, respectively.
These associations remained after accounting for traditional risk factors and other socio-environmental exposures like noise pollution and socioeconomic status.
In addition, these heart risks were higher among participants who lived in areas with additional social or environmental stress, such as high traffic noise or lower neighborhood income. Over a 10-year follow-up period, 17% of participants had major heart conditions.
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“We found a nearly linear relationship between nighttime light and heart disease: the more night-light exposure, the higher the risk. Even modest increases in night-time light were linked with higher brain and artery stress,” Abohashem said. “When the brain perceives stress, it activates signals that can trigger an immune response and inflame the blood vessels. Over time, this process can contribute to hardening of the arteries and increase the risk of heart attack and stroke.”
However, to counter the effects of artificial light at night, he suggested that cities could reduce unnecessary outdoor lighting, shield streetlamps or use motion-sensitive lights. On a personal level, “people can limit indoor nighttime light, keeping bedrooms dark and avoiding screens such as TVs and personal electronic devices before bed,” he said.
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“These findings are novel and add to the evidence suggesting that reducing exposure to excessive artificial light at night is a public health concern,” said Julio Fernandez-Mendoza, director of behavioral sleep medicine at Pennsylvania State University, Hershey.
The study had several strengths, including using state-of-the-art PET/CT imaging to measure brain stress and artery inflammation, combined with satellite light data and long-term follow-up for heart events. The CT/PET scans allowed them to see the inflammation coalesce.
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However, the study also has several limitations. Importantly, it is an observational study, an analysis of previously collected information; therefore, it cannot prove a direct cause-and-effect relationship between any of the variables reviewed. Second, the study included participants who received health care at only one hospital system, which means the participant group may not represent a diverse group of people, and the findings cannot be generalized to represent a larger population group.
“We know too much exposure to artificial light at night can harm your health, particularly increasing the risk of heart disease. However, we did not know how this harm happened,” Fernandez-Mendoza said.
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