Scientists Uncover Yet Another Reason to Sleep in Total Darkness

It turns out that your annoying friend who insists on closing all of the window blinds and covering all of the glowing electronics in a bedroom in order to sleep in total darkness is onto something. New research bolsters the well-established theory linking nighttime light to adverse health impacts.

A preliminary analysis set to be presented at the American Heart Association’s Scientific Sessions 2025, starting Friday, suggests that the state of darkness while we sleep plays a crucial role in our overall health. The results link more artificial light at night, also known as artificial nighttime light pollution, with higher brain stress signals, inflamed blood vessels, and greater risk of heart disease—a broad term for different heart problems.

“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,†Shady Abohashem, head of cardiac PET/CT imaging trials at Massachusetts General Hospital and senior author of the yet-unpublished study, said in a statement by the American Heart Association.

Artificial nighttime brightness at home

In the observational study, Abohashem and his colleagues reviewed the health data of 466 adults who had undergone the same combined Positron Emission Tomography/Computed Tomography (PET/CT) scan at Massachusetts General Hospital in Boston between 2005 and 2008 to identify stress signals in the brain and evidence of artery inflammation. They also investigated the adults’ exposure to artificial nighttime brightness at their homes via the 2016 New World Atlas of Artificial Night Sky Brightness.

The researchers found that people who experienced greater amounts of nighttime artificial light had higher brain stress activity, blood vessel inflammation, and a greater chance of developing heart disease. Unsurprisingly, the risk of heart issues was increased among participants who lived in areas with additional stress factors like significant traffic noise or lower neighborhood income. By the end of 2018, 17% of the adults had experienced significant heart problems.

“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 explained. The correlation remained even after researchers adjusted for known heart risk factors and other socio-environmental stresses.

“When the brain perceives stress, it activates signals that can trigger an immune response and inflame the blood vessels,†he added. “Over time, this process can contribute to hardening of the arteries and increase the risk of heart attack and stroke.â€

Put the phone down

So what can we do about it? Cities could diminish unnecessary external lighting, and individuals could lessen indoor nighttime light before going to bed, according to Abohashem. That includes screens, which means no more scrolling on TikTok before snoozing.

“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,†said Julio Fernandez-Mendoza, who is director of behavioral sleep medicine at Pennsylvania State University College of Medicine and was not involved in the study. “This study has investigated one of several possible causes, which is how our brains respond to stress. This response seems to play a big role in linking artificial light at night to heart disease.â€

However, the study has some limitations, according to the statement. For example, the participants were drawn from a single hospital system, so the group may lack diversity, and the results might not reflect the broader population. Furthermore, because of the nature of an observational study, it can’t prove that the associations are causal.

“We want to expand this work in larger, more diverse populations, test interventions that reduce nighttime light, and explore how reducing light exposure might improve heart health,†Abohashem concluded.