Sleeping with light in the room is linked to thicker, stiffer heart walls in a study of more than 11,000 people

Wrist sensors worn by more than 11,000 people turned an ordinary week of sleep into a dataset, and three years later, cardiac MRI scans on the same group showed a pattern researchers had not previously mapped in this much detail: hearts exposed to more light at night had thickened, stiffer walls and less room inside their chambers. Tulane University epidemiologist Lu Qi led the analysis, published in the European Heart Journal, comparing participants whose bedrooms registered above three lux of light overnight against those who slept in near-total darkness. The comparison points to bedroom light itself, not just poor sleep in general, as a measurable factor in how the heart is built.

Wrist Sensors Turn Bedrooms Into Data Points

The cohort came from UK Biobank, a research resource that has followed roughly 500,000 volunteers recruited between 2006 and 2010 to study why some people fall ill and others do not, sharing biological samples and health records with approved research teams worldwide. For this analysis, 11,071 of those volunteers wore light-and-motion sensors on their wrists for seven consecutive days sometime between 2013 and 2015, giving Qi’s team a week-long readout of exactly how dark, or how lit, each person’s sleeping environment actually was. Roughly three years after that measurement, the same participants underwent cardiovascular magnetic resonance imaging, which let the researchers examine heart structure directly rather than infer it from symptoms or hospital records.

Participants above the three-lux threshold showed measurable differences on those later scans, according to the paper posted on the European Heart Journal’s site and summarized separately in the original research release: roughly 2.4% greater left ventricular mass and 1.9% lower myocardial contraction fraction, alongside thickening in the left ventricle wall, reduced space inside the heart’s chambers, and decreased flexibility during each heartbeat. Changes also turned up in the right ventricle and left atrium, structures the researchers had not expected to see affected by something as simple as a lit hallway or a glowing television, inside a cohort whose underlying volunteer database was built for exactly this kind of long-range comparison.

A Pattern That Held Across Four Heart Structures

Qi described the scale of the association in direct terms. “Exposure to higher levels of light at nighttime is linked to cardiac remodelling,” Qi said, adding that the changes were consistent with a dose-response relationship, meaning more light tracked with more structural change rather than a single threshold effect. Sleep duration, the length of time participants actually spent asleep rather than how bright the room was, accounted for an estimated 24% to 49% of the association, leaving most of the effect tied to light exposure itself.

Over eight to ten years of follow-up after the imaging, participants in the high-exposure group faced meaningfully higher rates of downstream cardiac events: 29% more heart failure, 15% more atrial fibrillation, 24% more heart attacks, and 33% more strokes. Those follow-up figures come from the same UK Biobank cohort rather than a separate trial, so they describe an association observed over time in one population, not a controlled test of what would happen if someone’s nighttime light exposure were deliberately changed.

An Outside Cardiologist Calls Darkness a Vital Sign

Thomas Munzel of the University Medical Center Mainz, who was not involved in the study but wrote an editorial accompanying it, argued the findings should change how doctors take a history. “It is time for clinicians, particularly those managing patients with heart failure or atrial fibrillation, to start asking about the sleep environment: how dark the bedroom is, whether the patient works night shifts, and how much screen use occurs after sunset,” Munzel wrote. His suggested fixes were unglamorous: blackout curtains, warm-colored bedside lighting, and covering the small standby LEDs that glow on routers, chargers and television sets overnight.

Qi has been building toward this argument for several years, describing nighttime light exposure in earlier work as a factor tied to broader cardiovascular risk, and this study extends that line of research into the heart’s actual anatomy rather than just its downstream disease rates. “Light pollution has emerged as a new risk factor for cardiovascular disease,” Qi said, framing the reduction of nighttime light as a strategy clinicians and policymakers should weigh alongside more established interventions like blood pressure control.

Munzel went further than Qi in his editorial, calling darkness itself a metric worth tracking. “Darkness deserves recognition as a vital sign, as essential to cardiovascular health as blood pressure control and clean air,” he wrote. Neither researcher offered a specific lux target for bedrooms beyond the three-lux line used to define the study’s two comparison groups, leaving the practical question of how dark is dark enough for a follow-up study to settle.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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