A new study published in the European Heart Journal suggests that exposure to light during nighttime sleep may lead to subtle but potentially harmful changes in the structure and function of the heart. Earlier research had already linked nighttime light exposure to a higher risk of serious heart conditions, including coronary artery disease, heart attacks, and stroke. This study is the first to investigate how such exposure might physically alter the heart. These changes, known as cardiac remodeling, could help explain why too much light during sleep might increase the risk of heart problems, according to Dr. Lu Qi, lead researcher from Tulane University's Celia Scott Weatherhead School of Public Health & Tropical Medicine. The study analyzed data from 11,071 participants from the UK Biobank, a large-scale health research project that began in 2005. Participants wore a wrist-mounted device that measured light and movement for seven days. A few years later, they underwent MRI scans of their hearts. None of the participants had existing cardiovascular disease at the start of the study. Researchers compared individuals who slept in near-total darkness with those exposed to at least 3 lux of light—roughly the brightness of a dimly lit room. Common sources of nighttime light included streetlights coming through window shades, clock radios, and LED lights on devices like air purifiers or humidifiers. After adjusting for factors such as age, health, and lifestyle, the researchers found that any light exposure above 3 lux during sleep was linked to subtle but measurable changes in heart structure and function. Those in the highest light exposure group had a slightly larger left ventricle, a slightly thicker heart wall, and a slightly reduced ability of the heart to contract compared to those sleeping in the darkest rooms. While these changes may seem minor, they are known to be associated with an increased risk of heart disease and stroke. The study also found that people exposed to more nighttime light had a significantly higher risk of heart failure, heart attacks, stroke, and cardiovascular death over a 10-year follow-up period. Part of this risk may be due to disrupted sleep, as the study found that nearly half of the heart changes could be attributed to less sleep. Another possible cause is the effect of light on the body's circadian rhythm, which regulates a wide range of functions, including sleep, hormone release, and blood pressure. Disrupting this natural rhythm could lead to long-term cardiovascular issues. The study has some limitations. The light sensors were worn for only a week, and the devices measured brightness but not the type of light, such as LED or fluorescent. The study also included mostly white participants, so the findings may not apply to other racial groups. As it was observational, the study cannot confirm that nighttime light directly caused the heart changes. Future research could include people with existing heart conditions and examine the effects of different types of light and circadian rhythms. Despite these limitations, the study highlights simple steps people can take to reduce nighttime light exposure, such as using blackout curtains, warm-colored lighting, and covering standby lights on electronics. Cities could also help by using dimmer or motion-activated streetlights and shielding them to prevent light from entering buildings. These changes, the researchers suggest, may help protect heart health.