It’s well established that not getting enough sleep can raise your risk of heart disease, but a new study suggests that another bedtime factor may also harm the heart: light exposure.

The research, published in the European Heart Journal, found that even low levels of light at night were associated with a higher long-term risk of stroke, heart attack, and cardiovascular-related death. Risky changes to the heart’s structure also appeared connected to nighttime brightness.1

So does this mean you’d be better off without that lamp on your nightstand? Here’s what you need to know.

How Light Exposure Before Bed Affects Heart Structure

The study built on previous research linking nighttime light exposure to a higher risk of cardiovascular disease.2 Researchers from Tulane University wanted to see whether nighttime brightness was associated with not only adverse cardiac events, but also with actual changes to the heart’s structure.

For starters, the researchers collected data from more than 11,000 participants in the UK Biobank. The participants, who ranged in age from 40 to 69, were relatively healthy, with no diagnosed cardiovascular disease.

Between 2013 and 2015, participants wore wrist accelerometers equipped to detect ambient light and movement for seven consecutive days while they slept. They underwent heart scans three years later, and researchers followed them for another eight to 10 years.

The scientists then analyzed participants’ health outcomes in relation to light exposure during their five least active and 10 most active hours each day.

What the Researchers Found

The researchers found that exposure to even very dim light—3 lux or less—at bedtime was associated with structural and functional changes in the heart, as well as a higher long-term risk of heart failure, stroke, and other cardiovascular conditions. It was also linked to a higher risk of cardiovascular-related death.

That level of light is quite dim—even lower than the ambient light that might peek into a bedroom from a hallway nightlight, according to Rasi Wickramasinghe, MD, PhD, FACC, chief of cardiology at Houston Methodist Sugar Land Hospital.

“In the people with the most nighttime light,” Wickramasinghe added, “the findings weren’t a scattering of odd results, but a coherent pattern.” Their left ventricles were thicker and heavier, with roughly 2% more muscle mass and wall thickness.

In virtually every other muscle of the body, more mass is a good thing—but not so with the heart.

When the heart thickens, it can also stiffen, affecting its ability to pump properly. Overall, “it’s a sign of strain and stress on the heart,” Arash Bereliani, MD, FACC, staff cardiologist at Cedars-Sinai Medical Center and clinical director of Beverly Hills Institute of Cardiology and Preventive Medicine, told HEALTH.

Some Important Caveats

A word of caution: The study doesn’t definitively show that nighttime light causes heart problems.

Wickramasinghe noted that unmeasured factors associated with more light exposure, such as street noise or air pollution, could be responsible for some of the negative heart effects. Plus, he added, “One week of measurement is a snapshot of a lifelong habit.”

The study also didn’t specifically measure blue light, the type emitted by cell phones and other electronic devices. That makes it difficult to know whether blue light was more strongly associated with changes to the heart than other types of light.

Why Might Nighttime Light Be Linked to Heart Health?

Again, the study doesn’t prove that nighttime light causes the heart problems observed in the study. But there are reasons to think that nighttime brightness could affect the heart.

A major one is the well-established disruption that evening light exposure has on circadian rhythm, the internal body clock that helps coordinate blood pressure, heart rate, and other bodily functions.

When your circadian rhythm is out of sync, it can affect your sleep. In the study, shorter sleep duration accounted for 24% to 49% of the associations between nighttime light exposure and heart problems.

Even a few lux of light can suppress melatonin, potentially shortening and fragmenting sleep, Wickramasinghe said. Melatonin not only signals to the brain that it’s time for sleep, but it also helps the cardiovascular system by facilitating the normal 10% to 20% dip in blood pressure that happens overnight.3

Daytime light exposure also plays a big role in regulating the circadian rhythm. It suppresses melatonin during the day, helping with alertness. But the study found that daytime light exposure didn’t appear to “undo” the effects of nighttime light exposure.4

“Daytime light showed no association with cardiac structure or function,” Bereliani said.

Should You Avoid Light Exposure Before Bed?

It might be tough to block out all sources of light at night, especially if you live in a brightly lit neighborhood or share your home with other people. But you can take steps to minimize exposure.

Here’s what Bereliani suggests:

• Hang blackout curtains
• Cover any LED lights with dark tape
• Consider replacing a digital clock with an analog one
• Charge phones and devices outside of the bedroom or tucked away in a drawer
• Wear a comfortable, well-fitting eye mask to bed
• Place necessary nightlights close to the floor. An even better option is installing motion lights, which turn on only as needed.

But remember, a dark bedroom alone won’t prevent cardiovascular disease. “None of this replaces what we know moves the needle,” Wickramasinghe said, including exercise, having a balanced diet, not smoking, and managing blood pressure, cholesterol, and glucose levels.