Even the faint glow of a phone screen or a sliver of light under the door while you sleep may be doing more than disturbing your rest—it could be reshaping your heart. A study published in the European Heart Journal suggests that exposure to even modest levels of light at night is associated with structural changes in the heart, a condition known as cardiac remodelling.
“Exposure to higher levels of light at nighttime is linked to cardiac remodelling,” said Lu Qi of Tulane University in New Orleans, who led the research. “This is where the structure and function of the heart changes and we typically see it in response to chronic stress or injury to the heart.” While this is the body’s way of adapting to stress, he warned that “ultimately it weakens the heart and can lead to heart failure.”
The findings, published in the European Heart Journal, add to a growing body of evidence that light pollution is not just an environmental nuisance but a genuine cardiovascular risk factor. The authors argue that keeping bedrooms as dark as possible “should be considered as one of the potential strategies for preventing heart disease by clinicians and policy makers.”
What the study found
The research followed 11,071 participants who were exposed to varying levels of light overnight for a week—from near-total darkness to the equivalent of a low-brightness phone screen or light seeping through a cracked door. Three years later, cardiovascular magnetic resonance scans revealed that those exposed to the highest levels of nighttime light had thickened walls in the left ventricle, reducing the chamber’s internal space. There were also signs that the heart’s ability to flex during each beat had diminished—an early marker of dysfunction.
“Darkness deserves recognition as a vital sign, as essential to cardiovascular health as blood pressure control and clean air,” wrote Thomas Münzel of University Medical Center Mainz in Germany and colleagues in an accompanying editorial. They urged clinicians to ask patients about their sleep environment: “how dark the bedroom is, whether the patient works night shifts, and how much screen use occurs after sunset.”
Practical advice from the editorial includes using blackout curtains, opting for warm-coloured bedside lighting, and covering the small standby LEDs that dot household electronics. These simple measures, they argue, could be as important as monitoring blood pressure.
Beyond the bedroom: cities and light pollution
The authors also point to a broader role for municipalities. Light pollution is “one of the few environmental hazards that cities can control directly and affordably,” they note. Shielded streetlight fixtures, warm-spectrum bulbs, and dimming or motion-activated lighting are described as “energy-efficient, and climate-friendly” ways to reduce exposure. Several European cities, including Paris and Berlin, have already begun experimenting with such measures, and the new findings may give further impetus to those efforts.
The study’s implications extend beyond individual bedrooms. In densely populated European countries, where night-time illumination is pervasive, the findings suggest that urban planning and lighting policy could have a measurable impact on public health. As Europe's carbon footprint from imports remains a concern, reducing unnecessary lighting also aligns with climate goals.
While the study is observational and cannot prove causation, the authors stress that the link is biologically plausible. Light at night suppresses melatonin, a hormone that regulates sleep and has antioxidant properties, which may contribute to cardiovascular stress. The findings echo earlier research on lifestyle factors and disease risk, reinforcing the idea that small daily habits can have long-term health consequences.
For now, the message is clear: if you want to protect your heart, make your bedroom as dark as possible. And if you live in a city, consider advocating for smarter, less intrusive street lighting. As the editorial puts it, darkness is not just for sleep—it’s a health necessity.
