Nighttime light exposure may be linked to changes in the structure and function of the heart, according to a new study that found greater exposure during sleep was associated with thicker heart muscle and early signs of impaired cardiac function. The findings suggest that even relatively modest amounts of light in the bedroom could be relevant to cardiovascular health.
The study, published September 9, 2026, in the European Heart Journal, examined more than 11,000 UK Biobank participants who did not have cardiovascular disease at the beginning of the analysis. Researchers measured nighttime light exposure with wrist-worn devices and later used cardiovascular magnetic resonance imaging to examine the participants’ hearts.
The researchers focused on nighttime exposure above 3 lux. Lux measures illuminance, or the amount of visible light reaching a surface. In the study, exposure above this threshold was associated with changes across several heart chambers, including increased left ventricular mass and wall thickness and reduced measures of myocardial contraction.
The findings do not establish that bedroom light directly causes heart disease. Instead, they identify an association between nighttime light exposure and cardiac remodeling, while also suggesting that shorter sleep duration may explain part of the relationship.
European Heart Journal study on nighttime light and cardiac structure provides the full methodology and results.
Nighttime Light Exposure Was Associated With Heart Remodeling
The study included 11,071 participants who wore an accelerometer with an integrated light sensor for seven consecutive days between 2013 and 2015. The researchers then examined cardiovascular magnetic resonance scans obtained about three years later to assess the structure and function of the heart.
The analysis found that people exposed to more than 3 lux of nighttime light had a greater left ventricular mass and thicker average heart-muscle walls than participants without that level of exposure. The study also found a lower myocardial contraction fraction, a measure that reflects how effectively the heart muscle contracts.
The differences were measurable but relatively small. Compared with no nighttime exposure above 3 lux, high exposure was associated with a 2.4% greater left ventricular mass indexed to height, a 1.5% greater mean wall thickness and a 1.9% lower myocardial contraction fraction.
Researchers also identified changes in the way the heart muscle deformed during contraction. Measures of circumferential, radial and longitudinal strain were lower among participants with greater nighttime light exposure, indicating less effective myocardial deformation.
The effects were not limited to the left ventricle. Greater nighttime light exposure was also associated with changes in the right ventricle and left atrium, including larger indexed ventricular volumes and a lower left atrial ejection fraction. The researchers described the overall pattern as adverse cardiac remodeling and subclinical functional decline across multiple chambers.
Cardiac remodeling refers to changes in the size, shape or structure of the heart muscle that can occur in response to stress or other cardiovascular conditions. Thickening of the heart muscle can reduce the chamber’s available space and alter how efficiently the heart fills and contracts.
The study also found that the relationship followed a dose-response pattern. In general, greater nighttime light exposure was associated with greater changes in cardiac measurements, rather than the findings being limited to a simple exposed-versus-unexposed comparison.
Shorter Sleep May Explain Part of the Association
One important finding was the role of sleep duration. The researchers estimated that shorter sleep statistically mediated between 24% and 49% of the associations between nighttime light exposure and the cardiac measurements they examined.
That does not mean light exposure alone accounts for all of the observed differences. Instead, the results suggest that disrupted or shortened sleep could be one pathway connecting nighttime light with changes in heart structure and function.
The American Heart Association already identifies healthy sleep as an important component of cardiovascular health and recommends that adults generally aim for seven to nine hours of sleep. It also advises keeping the bedroom dark and reducing exposure to bright screens and other light sources before bed.
American Heart Association guidance on sleep and heart health outlines practical measures for improving the sleep environment and maintaining healthier sleep habits.
The new findings add to a growing body of research examining the relationship between nighttime light and cardiovascular health. Earlier research has associated artificial light at night with cardiovascular risk factors, while laboratory work has shown that moderate light exposure during sleep can alter heart rate, heart-rate variability and next-morning insulin sensitivity.
The researchers also examined longer-term cardiovascular outcomes in a much larger group of UK Biobank participants. During eight to 10 years of follow-up, higher nighttime light exposure was associated with increased risks of heart failure, atrial fibrillation, myocardial infarction, stroke and cardiovascular mortality. Those findings were observational associations and therefore do not establish that light exposure itself caused the outcomes.
UK Biobank’s overview of its health and research data explains the large-scale database used for this research and its longitudinal health information.
How to Reduce Light Exposure While Sleeping
Reducing nighttime light is relatively straightforward when the bedroom contains obvious sources of illumination. Researchers and sleep experts recommend limiting unnecessary light from phones, digital clocks, hallway lighting and outdoor sources.
Blackout curtains or shades can help prevent streetlights and other exterior illumination from entering through windows. Opening the curtains in the morning can then provide natural daylight, which helps reinforce the body’s circadian rhythm.
Small sources of indoor light can also matter. A smartphone left face-up beside the bed, an illuminated alarm clock or light entering around a bedroom door can contribute to nighttime exposure. Turning off unnecessary lights and covering bright electronic indicators can reduce those sources.
If some light cannot be eliminated, a sleep mask can provide another way to reduce the amount reaching the eyes. For people who need a bedside or nighttime light, dim illumination positioned low to the floor and warmer-colored light may be preferable to bright, cool lighting.
The distinction between nighttime and daytime light is important. Light is the primary environmental signal that helps synchronize the body’s circadian clock, and morning exposure to natural light can support a regular sleep-wake rhythm. The American Heart Association has emphasized that nighttime light can disrupt circadian timing, while appropriate light exposure during the day can reinforce it.
The study’s researchers cautioned that their findings require further investigation. Participants wore the light-sensing device for only one week, which may not fully represent their long-term exposure patterns. In addition, a wrist-mounted light sensor does not precisely measure the amount of light reaching the eyes.
Those limitations are important because the study demonstrates associations rather than proving causation. Further research will be needed to determine whether reducing nighttime light exposure can prevent or reverse the cardiac changes identified in the study.
PubMed research on light exposure during sleep and cardiometabolic function provides additional evidence on how nighttime illumination may affect cardiovascular and metabolic responses during sleep.
