Many people sleep with a night-light their whole lives—though as they grow older, small plug-in light fixtures may be replaced by the glow of laptops or phone screens. While sleeping in a low level of light may be commonplace, doing so comes with its own health hazards, including altering the shape of the human heart.
A new study, published this week in the European Heart Journal, links exposure to light while sleeping to cardiac remodeling, or changes to the shape and function of the heart. Specifically, light exposure during sleep led to thickening of the wall of the left ventricle, one of the organ’s four chambers, reducing the space inside the heart. That weakens its ability to flex during a heartbeat, setting the stage for cardiac dysfunction.
“Light pollution has emerged as a new risk factor for cardiovascular disease,” Dr. Lu Qi, a professor at Tulane University and an author of the study, said in a press release. “Our findings, together with evidence from other studies, suggest that reduction of nighttime light exposure should be considered as one of the potential strategies for preventing heart disease by clinicians and policymakers.”
How much light is too much light?
To determine how light exposure during sleep impacts cardiovascular health, the study’s researchers analyzed the sleep habits of 11,071 participants. Subjects wore a light sensor on their wrists while sleeping for seven days. Three years later, their hearts were scanned via cardiac MRIs to assess their structure and function.
Those who were exposed to more than 3 lux overnight saw significantly more cardiac remodeling compared with those who slept in near complete darkness. Lux, the International System unit for light exposure, measures how much light falls on a given surface area.
Three lux, the study’s benchmark for a high level of light exposure, may not seem like much light at all. It’s equivalent to the brightness of deep twilight, or to the amount of light that bleeds under a door from a bright adjacent room.
A phone screen, meanwhile, can top out around 80 lux, according to experts. With 58% of U.S. adults saying they use their phone within an hour of bedtime or while in bed as they fall asleep, that’s more than half of Americans who may be unknowingly setting themselves up for heart problems in the future.
How to sleep more safely
To avoid light exposure during sleep, experts recommend using blackout curtains or sleep masks and covering small standby lights on electronics. Limiting screen time immediately before bed (and especially while falling asleep) is important, too. For the best quality sleep, ambient light should be limited to a maximum of just 1 lux, which is close to total darkness.
In an article accompanying the study, cardiologist Thomas Münzel and his colleagues spoke to how its results might inform the future of medicine, including the questions doctors ask their patients during health screenings.
“It is time for clinicians, particularly those managing patients with heart failure or atrial fibrillation, to start asking about the sleep environment: how dark the bedroom is, whether the patient works night shifts, and how much screen use occurs after sunset,” Münzel wrote. “Darkness deserves recognition as a vital sign, as essential to cardiovascular health as blood pressure control and clean air.”