There may be a really simple way to help patients lower their risk for cardiovascular disease: getting them to sleep with the lights off.

In a new study, the largest of its kind, researchers analyzed light exposure records from nearly 90,000 people — 13 million hours’ worth of data — and compared light exposure levels to cardiovascular disease records collected 8-10 years later. People who slept in the brightest rooms had a 30%-50% higher risk than those who snoozed in darkness.

The fix is so simple, your patients could start to implement it tonight.

“For a lot of other approaches, you could say, well, because it’s so complicated and there are a lot of pros and cons, you need stronger evidence,” said Frank Scheer, PhD, director of the Medical Chronobiology Program at Brigham and Women’s Hospital in Boston and one of the co-authors of the study. “For something like light exposure at night, there’s no downside to keeping it dark other than using a dim light for safely getting out of bed.”

The Effects of Individual Light Exposure

The study, “Light Exposure at Night and Cardiovascular Disease Incidence,” was published in JAMA Network Open. It drew on data collected from 89,000 participants in the UK Biobank (average age, 62; 57% female; and 97% White) who wore light-tracking devices on their wrists for 1 week in the 2010s. Researchers grouped their light exposure profiles into percentile cohorts: 0 to 50th, 51st to 70th, 71st to 90th, and 91st to 100th, and tracked the cohorts’ rates of cardiovascular conditions.

People exposed to the brightest lights at night — the equivalent of sleeping with overhead lights on — were 56% more likely to develop heart failure, 47% more likely to have a heart attack, and roughly 30% more likely to develop coronary artery disease, atrial fibrillation, or stroke than those in the lowest percentile group. Those results held even after accounting for cardiovascular risk factors, including diet, exercise, smoking, and genetics.

“We thought there’d be some effect, but to see close to a 60% increased risk of heart failure with bright night light, the strength of the effect was surprising,” said Daniel P. Windred, PhD, lead author and research associate at Flinders University’s FHMRI Sleep Health in Adelaide, Australia. “We’re starting to get up there with other risk factors in terms of how significant it is.”

Not only was there a significant effect, but researchers also found a dose-response relationship. The brighter the room, the more likely people were to experience cardiovascular disease. But it didn’t take much light to produce an effect.

“Some research from our group showed people’s circadian system can be reset by light almost as dim as candlelight — there’s a large difference between people,” Windred said. “Almost any light may affect you.”

Possible Mechanisms

Because the research is observational, researchers couldn’t determine why light exposure during sleep has these effects, but the link between the circadian system and cardiovascular function is well established. Scheer has been studying it for decades.

The American Heart Association published a scientific statement in October on the role of the circadian system in cardiometabolic health. Among other findings, it noted that circadian disruption affects blood pressure regulation and hypertension risk, and fragmented circadian rhythms increase the risk for cardiovascular disease.

“Our thinking is that the light disrupts circadian rhythms, which leads to a dysregulated physiological state, and if you disrupt these rhythms over a long enough period of time, chronically, you possibly elevate people’s risk of cardiovascular diseases,” Windred said.

Easy Advice for Patients

As there’s no downside to sleeping in a dark room, you can start sharing these discoveries with your patients immediately, both Windred and Scheer said. They suggested incorporating it into everyday discussions.

“If you have somebody who’s at risk for cardiovascular disease or somebody who has insomnia, asking patients about their light-at-night exposure and recommending to keep it dark might be especially important,” Scheer said.

Once you’ve had that brief talk, this may be the simplest, most likely-to-be-followed advice you’ll ever offer your patients:

Turn out (almost all) the lights. No matter if it’s a bedside lamp, a streetlight, or the glow of a TV screen, sleeping with light in your eyes may raise your risk for heart disease.

“It doesn’t necessarily need to be pitch-dark,” Scheer said. “When you think about the circadian system, we know what intensity of light people’s clocks respond to. If you keep light really dim, below five lux, which is the equivalent of five candles at arm’s length, you can still see your surroundings, but it has minimal effect on your circadian system.”

For those who find it comforting to fall asleep with the television on, he suggested switching to the radio or a podcast — a boring one.

Choose your dim light carefully. A night light is fine, Windred said, but advise patients to select one that leans toward an orange glow. Bluer lights have a more pronounced cardiovascular effect.

Use blackout curtains. If a patient’s bedroom faces the street, external light may pose a problem. Heavy curtains can solve it.

Consider a sleep mask. For patients whose partners don’t stick to the same sleep schedule, a comfortable sleep mask may help.

“It’s a very easy modification to make in your life,” Scheer said. “It’s not a big investment, it doesn’t cost anything, you’re not giving up a lot. The risk/benefit of all this is very attractive — it’s scalable, it applies to everybody. There’s hardly an easier way to improve your health than to make this tiny modification.”