More than 3 lux of bedroom light during sleep was tied to thicker heart muscle walls and weaker pumping function in over 11,000 adults without prior cardiovascular disease.
The study tracked nighttime light with wrist sensors for seven days and found damage on cardiac MRIs three years later, with risk rising as bedrooms got brighter.
Researchers said 24% to 49% of the effect was explained by shorter sleep of under five or six hours, while stress and changes in blood pressure or heart rate may also contribute.
Outside experts said the findings show an association, not proof of causation, citing the short measurement window and wrist sensors as imperfect proxies for light reaching the eye.
The paper adds to evidence that limiting nighttime light—such as phone glow, alarm clocks and outdoor spillover—could become part of heart-disease prevention advice.