Sleeping with the lights on linked to early signs of heart damage

The study, published in the European Heart Journal, found that people exposed to nighttime light levels above just 3 lux, roughly the brightness of moonlight or a sliver of light under a closed door, showed measurable changes in their heart muscle compared to those who slept in complete darkness.

Researchers from Tulane University tracked more than 11,000 participants from the UK Biobank, a British biomedical database holding genetic and health data from half a million residents.

Each participant wore a wrist sensor for seven days to record their light exposure at night. Three years later, they underwent cardiovascular MRI scans to assess the structure and function of their hearts.

The results showed that those exposed to higher levels of nighttime light had a 2.4 percent greater left ventricular mass, a 1.5 percent thicker heart wall on average, and a 1.9 percent drop in the heart's contraction efficiency. Additional changes were also observed in the right ventricle and left atrium.

Lead author Dr Lu Qi, director of the Tulane University Obesity Research Center, described the process as cardiac "remodeling," a shift in the heart's shape and function typically triggered by chronic stress or injury. "It's our body's way of adapting to that stress, but ultimately it weakens the heart and can lead to heart failure," Qi said.

The changes recorded in the study were small and did not amount to diagnosable heart disease. But researchers say they could be early signals of a process that, left unchecked, raises the risk of conditions such as atrial fibrillation and heart failure over time.

According to Qi, between 24 and 49 percent of the effect could be explained by shorter sleep duration linked to nighttime light exposure, with the rest potentially tied to disrupted stress hormones and other biological changes.

Cardiologist Dr Cheng-Han Chen, who was not involved in the study, said the findings build on a growing body of evidence linking nighttime light to high blood pressure, high cholesterol, obesity and diabetes.

"Nighttime light may also disrupt the body's natural circadian rhythm, which plays an important role in regulating blood pressure, metabolism, sleep and healthy heart function," Chen said.

The findings echo a separate 2025 study of nearly 89,000 UK Biobank participants, which found that those exposed to the brightest nighttime conditions faced significantly higher risks of coronary artery disease, heart attack, heart failure, atrial fibrillation and stroke compared to those who slept in the dark. Experts caution that the research is observational, meaning it shows an association rather than direct proof that light itself causes heart damage.

Karl Lawrence, a pharmaceutical science lecturer at King's College London, noted that a single week of light tracking is a short window to draw firm conclusions from, and called for further studies to establish causality.

Doctors say the good news is that cutting nighttime light exposure is relatively easy.

Recommended steps include using blackout curtains or blinds to block outdoor light, turning electronics away from the bed and covering indicator lights, and keeping phones face down or out of the bedroom altogether.

For those who cannot guarantee full darkness, a sleep mask offers a simple workaround, while any necessary night lighting should lean toward warm, dim, reddish tones rather than harsh white light. Getting plenty of natural light during the day also helps reinforce the body's circadian rhythm.

For those unable to fully eliminate nighttime light, experts say focusing on other controllable risk factors, such as eating a heart healthy diet, exercising regularly, maintaining a healthy weight, and avoiding tobacco and excessive alcohol, remains key to protecting heart health.