If you sleep with a TV glowing, a phone nearby or a streetlight leaking through the blinds, your heart may be paying a quiet price.
New research links nighttime light to thicker heart walls and weaker pumping.
The study, published September 9, 2026, in the European Heart Journal, followed 11,071 UK Biobank adults with an average age of 61.
Each wore a wrist sensor for a week to measure how much light reached them at night. Cardiac MRI scans about three years later showed the changes.
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Compared with adults who slept in near darkness, those exposed to more than 3 lux of light at night had 2.4% greater left ventricular mass, 1.5% thicker walls and roughly 1.9% weaker muscle contraction, the study found.
Reduced flexibility also appeared in the right ventricle and left atrium.
Together, these changes fit a pattern doctors call cardiac remodeling, meaning the heart's structure and function shift in response to chronic stress.
What the numbers meant for heart disease later
The team also tracked cardiovascular events over 8 to 10 years in a larger group of more than 71,000 participants.
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High nighttime light exposure was tied to a 29% higher risk of heart failure and a 33% higher risk of stroke.
Heart attack risk rose 24%, atrial fibrillation 15%, and cardiovascular death 26%.
Shorter sleep duration explained 24% to 49% of the link.
Part of the harm appeared to come from light directly, and part from the shorter, more fragmented sleep it caused.
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What researchers recommend
For quality sleep, the researchers said the bedroom should be as dark as possible, ideally no brighter than 1 lux, which is dimmer than a full moon.
Most bedrooms with a screen or a streetlight nearby sit well above that.
Practical steps flagged in the paper and an accompanying editorial include blackout curtains, warm-toned bedside lamps and covering the small standby LEDs on TVs, chargers and appliances.
Editorial author Professor Thomas Münzel of the University Medical Center Mainz urged clinicians, especially those treating heart failure or atrial fibrillation patients, to start asking about their sleep environment.
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Because the research is observational, it can show a strong pattern but cannot prove that ambient light alone causes heart damage.
The size of the group, the objective wrist-sensor readings and the follow-up MRI scans make it one of the more rigorous efforts on the topic so far, and the results echo earlier findings tying light at night to poor cardiometabolic outcomes.
Anyone with a known heart condition or symptoms such as breathlessness or an irregular pulse should raise those concerns with a doctor rather than adjust treatment on their own.
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This article is made and published by Ida-Marie Palm Varbæk, who may have used AI in the preparation.