Sleeping with even a small amount of light in the room may raise the risk of heart disease, according to a new study led by researchers at Tulane University in Louisiana.
The researchers analyzed the sleep habits of 11,017 people without heart disease, all of whom wore a wrist monitor with a built-in light sensor for one week to measure how much light they were exposed to at night. Roughly 25 percent of participants had no nighttime light exposure above three lux.
About three years later, each participant underwent an MRI scan of the heart. The scans showed that people who had slept with more than three lux of light, roughly the brightness of a bright moonlit night, had thicker walls in the heart's left ventricle, along with greater wall mass and greater strain on that part of the heart.
The study also found greater cardiac chamber volumes in the left and right ventricles and the left atrium among those exposed to more light at night.
Why the left ventricle matters
The left ventricle is responsible for pumping oxygen-rich blood to the rest of the body. When its walls thicken, there is less space inside the chamber, forcing the heart to work harder. Researchers say this raises the risk of heart attack, rhythm disorders and stroke.
Patients exposed to more nighttime light also had heart muscles that were less able to flex during a heartbeat, a sign researchers describe as "cardiac remodeling," or structural changes that reduce the heart's ability to beat and sustain pressure.
The damage grew worse in brighter rooms. Common sources of nighttime light exposure included the glow of a smartphone, light from a digital alarm clock and bright lights from outside the home.
Shorter sleep plays a role
The researchers estimated that between 24 and 49 percent of light's impact on the heart was due to participants sleeping less, averaging about five to six hours a night.
Dr Lu Qi, senior study author and director of the Tulane University Obesity Research Center in New Orleans, said light pollution has emerged as a new risk factor for cardiovascular disease. He said the findings, together with evidence from other studies, suggest that reducing nighttime light exposure should be considered by clinicians and policymakers as one potential strategy for preventing heart disease.
In an email to CNN, Qi said shorter sleep duration was a direct consequence of night light exposure, and that night light exposure may also affect other cardiovascular disease risk factors such as stress and biological changes.
Study details and limitations
The study, published in the European Heart Journal, examined 11,017 patients between the ages of 40 and 69 recruited from the UK Biobank database. The average age of participants was 61, and 57 percent were women. Ninety-eight percent of participants were white.
The researchers acknowledged limitations, including that light exposure was measured for only one week using a wrist sensor rather than a sensor measuring light exposure directly on the eyes.
The findings come as nearly a million Americans die of heart disease each year, making it the leading cause of death in the United States.
What researchers recommend
The researchers urged people to limit nighttime light exposure to under one lux, describing this as a very dim room lit by no more than a single candle several feet away. They recommended tools such as blackout curtains and turning off hallway lights that may creep into the bedroom.
