Nighttime Light Exposure Is Linked to Heart Changes

For most of human history, night has been dark. Very, very dark—no hall light on, no streetlights streaming through the windows, no TV casting ambient light in the next room. Now, however, it’s rare to find yourself in total darkness. Recent research has found that might be a problem. Light exposure at night has been linked to higher rates of diabetes, obesity, and hypertension. Last year, researchers connected it to higher rates of heart failure and other cardiovascular problems over 10 years. Publishing in the European Heart Journal on September 9, another research team now reports that compared to people sleeping in darkness, people exposed to light at night tend to have structural changes in their hearts. The changes indicate that the organ is working harder than usual, the researchers say, providing further support for the idea that nighttime light exposure is connected to worse cardiovascular outcomes.How does light at night affect health?This study drew on data from fitness monitors that were worn for a week by a subset of participants in the UK Biobank, which collects health and lifestyle information on half a million people. The monitors contain a light sensor, which allows for an objective measurement of light exposure, says Dr. Lu Qi, a professor of epidemiology at Tulane University and an author of the new study. “That's very unique, because most previous studies have used self-reported light exposure,” he says. The other unique feature of the study involved cardiac MRIs that were taken of peoples’ hearts after they had worn the fitness monitors but before they developed any diagnosable heart problems. After sorting the participants into those with minimal light exposure at night, and those who routinely slept with at least 3 lux of light streaming over them (the equivalent of roughly three candles held three feet away), the researchers examined these scans for structural and functional changes, like the thickness of the heart’s walls and the movement of the valves that keep blood pumping.They found that the hearts of people exposed to nighttime light had thicker walls and were less effective at pumping. These changes do not themselves indicate heart disease, but precede its development, says Qi. These results—correlational as they are—add greater detail to the picture sketched by earlier work finding a correlation between light at night and a higher risk of heart disease, says Angus Burns, an instructor in medicine at Harvard Medical School who co-wrote a paper last year on the trend. “They show the same associations of light at night with stroke, myocardial infarction, heart failure,” he says. “The place where it really extends the literature is the MRI scans of the heart, which was fantastic.” How much light is too much light?With this kind of study, it’s impossible to eliminate the possibility that these heart problems and light exposure are both linked with another unknown factor in these people’s lives. But overall, the research suggests that light exposure at night may be a driver of metabolic and cardiovascular problems. It’s not entirely clear what was causing the light exposure seen in the data—whether it was lights on elsewhere in the house, lights on in the room, a tablet or other screen on nearby, or perhaps a television left on. But for the moment, the best rule of thumb is to get the room as dark as possible, these studies suggest. It’s also long been a core tenet of sleep hygiene. Turn off the hall light. Put up blackout curtains. Draw an eye mask over your eyes—and know that it could be helping your heart.