Sleeping with even relatively low levels of light in the bedroom may be associated with changes in the structure and function of the heart, according to new research that adds to growing evidence linking nighttime light exposure with cardiovascular health.

The study, published Sept. 9 in the European Heart Journal, examined thousands of adults enrolled in the UK Biobank and found that people exposed to more light during the nighttime hours showed signs of cardiac remodeling, including thicker heart walls and subtle declines in heart function. Researchers also found that greater nighttime light exposure was associated with a higher long-term risk of several cardiovascular conditions.

The findings come as artificial light has become nearly unavoidable for many people, whether from televisions, smartphones, bedside lamps, streetlights or illuminated buildings outside bedroom windows.

“Previous observational studies have linked nighttime light exposure with a higher risk of cardiovascular disease, but little is known about the related cardiac structural and functional changes,” study leader Lu Qi, a professor at Tulane University School of Public Health and Tropical Medicine, said in a statement released by the European Society of Cardiology.

Researchers analyzed 11,071 UK Biobank participants who wore wrist devices equipped with light sensors for seven days. The devices measured participants’ exposure to light at night. About three years later, the participants underwent cardiovascular magnetic resonance imaging, or cardiac MRI, allowing researchers to examine the heart’s chambers, walls and pumping function.

The researchers focused in part on nighttime exposure above three lux, a relatively low level of illumination. Compared with participants with essentially no exposure above that level, those with high nighttime light exposure had about 2.4% greater left ventricular mass, 1.5% greater average wall thickness and a 1.9% lower myocardial contraction fraction, a measure related to how effectively the heart muscle contracts.

Changes were also detected in the right ventricle and left atrium, suggesting that the association was not confined to a single part of the heart.

Why nighttime light may matter

Light is one of the body’s most important signals for regulating circadian rhythms, the roughly 24-hour biological cycle involved in sleep, hormone production, blood pressure, heart rate and metabolism. Artificial light at night can interfere with those signals and may also shorten or disrupt sleep.

In the new study, shorter sleep duration appeared to statistically explain between 24% and 49% of several associations between nighttime light exposure and changes in the heart. That does not mean sleep was the only mechanism involved, but the result suggests that interrupted or shortened sleep could be part of the connection.

Earlier experimental research has also found measurable effects from nighttime illumination. In a small laboratory study of healthy adults, one night of sleeping under moderate room light increased nighttime heart rate, reduced heart-rate variability and increased insulin resistance the following morning compared with sleeping in very dim light.

The latest European Heart Journal study went further by examining clinical outcomes in a larger group of 73,286 people. During approximately eight to 10 years of follow-up, participants with high nighttime light exposure had a 29% higher risk of heart failure, 15% higher risk of atrial fibrillation, 24% higher risk of heart attack, 33% higher risk of stroke and 26% higher risk of cardiovascular death compared with participants who had no nighttime exposure above the study threshold.

Those figures represent associations rather than proof that nighttime light directly caused heart disease. The study was observational, and factors related to where people live, work schedules, existing health conditions or other behaviors could also influence cardiovascular risk.

Still, the findings are consistent with other research connecting bedroom light with sleep health. A U.S. study involving nearly 48,000 women found that sleeping with a television on was associated with higher rates of short sleep, insomnia symptoms and inconsistent sleep schedules. Researchers reported that some measures of poor sleep associated with indoor nighttime light disproportionately affected Black women.

That broader context is important for health access because sleep conditions are not determined solely by individual choices. People working overnight or rotating shifts, living near bright roadways or commercial areas, or residing in housing where outside light is difficult to block may have less control over their nighttime environment. The National Institutes of Health has also identified persistent differences in sleep duration across racial, ethnic and socioeconomic groups, underscoring the need to consider sleep as both an individual and public health issue.

Researchers say relatively simple steps may help reduce nighttime exposure, including keeping unnecessary lights and televisions off, limiting bright screens near bedtime and blocking intrusive outdoor light when possible.

Qi said the findings suggest reducing nighttime light exposure could become one potential strategy for cardiovascular disease prevention.

The study does not establish that making a bedroom darker will prevent heart disease. But together with earlier evidence on sleep, circadian rhythms and cardiovascular health, it suggests that the environment people sleep in may deserve more attention alongside familiar factors such as sleep duration, exercise, diet and blood pressure.

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