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Darker Nights, Better Heart Health? New Study Weighs In
In A Nutshell
- People with high nighttime light exposure had a 29% higher risk of heart failure and a 33% higher risk of stroke than those with none, in a study of more than 70,000 adults.
- MRI scans of about 11,000 participants showed nighttime light tied to thicker heart walls, larger heart chambers, and weaker pumping function.
- Shorter sleep explained only 24% to 49% of the heart changes, leaving room for other possible pathways like circadian disruption.
- The study is observational, so it cannot prove light directly damages the heart, though researchers ruled out several alternative explanations.
People who slept with more light in their rooms went on to develop a 29% higher risk of heart failure and a 33% higher risk of stroke than those who slept in near-total darkness, according to a study of more than 70,000 adults.
Researchers used wrist-worn sensors to measure light exposure at night, then scanned some participants’ hearts with MRI three years later. Those with the most nighttime light had thicker heart walls, larger heart chambers, and hearts showing subtle signs of reduced contraction and poorer mechanical function. Follow-up health records from a larger group also showed higher rates of heart attack, irregular heartbeat, and death from heart disease.
This is the first study, published in the European Heart Journal, to connect nighttime light exposure to physical changes inside the heart, rather than just blood pressure or cholesterol numbers. Similar changes have been linked in previous research to a higher risk of later cardiovascular disease. Light pollution in the bedroom isn’t just an annoyance, this research suggests. It may be a genuine risk factor for heart disease.
Wrist Sensors Tracked a Week of Nighttime Light
Scientists drew on data from the UK Biobank, a long-running UK health study. Between 2013 and 2015, participants wore a small wrist device, similar to a fitness tracker but fitted with a light sensor, for a full week. The device recorded how much light hit the wrist around the clock, letting researchers calculate how much time each person spent above 3 lux, a threshold linked to suppressed melatonin release, during their least active hours, used as a stand-in for nighttime.
About 11,000 of those participants later underwent a cardiac MRI, roughly three years after tracking ended. This group averaged 61 years old, was about 57% female, and was almost entirely White, matching the larger UK Biobank study. People with existing heart disease at the start were excluded, so results reflect people with no diagnosed heart condition to begin with.
Researchers sorted participants into groups, from none up through low, moderate, and high nighttime light exposure, adjusting for age, sex, weight, smoking, exercise habits, income, air pollution, and conditions like diabetes and high blood pressure. A larger group of 73,286 participants was followed for eight to ten years to see who developed heart failure, an irregular heartbeat called atrial fibrillation, heart attack, stroke, or death from heart disease.
High Light Exposure Left Hearts Thicker and Weaker
People in the high-exposure group showed several warning signs linked to future heart trouble in other research. Compared with people who had no nighttime light exposure, they had about 2.4% greater heart muscle mass and 1.5% greater wall thickness, consistent with the heart’s main pumping chamber thickening in an unhealthy way. Their hearts also showed roughly 1.9% lower contraction efficiency, meaning the muscle didn’t squeeze and stretch with each beat quite as well as it should.
Changes weren’t limited to one part of the heart. Researchers also found mild enlargement in the right side of the heart, plus enlargement and reduced pumping function in the upper-left chamber, called the left atrium. Every pattern moved the same direction: more nighttime light meant progressively worse heart measurements, not a sudden jump at some threshold.
Disease outcomes were harder to ignore. Beyond the heart failure and stroke risk mentioned, the high-exposure group also had a 24% higher risk of heart attack, a 15% higher risk of atrial fibrillation, and a 26% higher risk of dying from cardiovascular disease, compared with people with no nighttime light exposure.
Part of the puzzle, though not all, traced back to sleep. People with more nighttime light exposure tended to sleep less, and shorter sleep accounted for 24% to 49% of the various heart changes, depending on the measurement. Much of the association stays unexplained by shorter sleep alone. Circadian disruption, the nervous system, or hormones are possible additional pathways, but this study could not establish them.
Bright daytime light showed almost none of these links. That contrast strengthens the case that something specific about light during the body’s normal rest period, not light exposure in general, is driving the pattern.
Darker Nights Emerge as a Modifiable Heart Risk
This research cannot prove nighttime light directly damages the heart, since it’s observational, tracking people over time rather than testing a change and measuring the result. People exposed to more light at night may differ from those who aren’t in ways researchers couldn’t fully measure. Still, the team took steps to rule out early, undiagnosed heart problems as the real explanation, and the pattern held up.
Dark nights and adequate sleep appear to matter for heart health beyond feeling rested the next morning. Keeping nights darker, whether by blocking streetlight, powering down electronics, or shutting off hallway fixtures, is a simple habit consistent with the findings, although this research does not show that dimming the bedroom will prevent heart disease. Given how common nighttime light exposure has become, that’s a pattern worth watching rather than dismissing as just another wellness trend.
Disclaimer: This article summarizes findings from a peer-reviewed observational study and is intended for general informational purposes only. It is not medical advice and should not be used to diagnose, treat, or make decisions about any health condition. Anyone with concerns about heart health or sleep habits should speak with a qualified physician.
Paper Notes
Limitations
The study’s authors note that light exposure was measured for only one week, which may not perfectly capture a person’s long-term habits, though they point out that light patterns tend to stay fairly stable over time in this population. The wrist-worn sensor recorded light reaching the device rather than light reaching a person’s eyes while asleep, and it could not measure the specific wavelength or color of light, so the researchers could not pinpoint whether certain types of light, such as blue light, mattered more than others. The study also could not account for individual differences in how sensitive people’s eyes and bodies are to light. Because the design was observational, cause and effect cannot be fully established, and reverse causation, meaning early undetected heart problems affecting nighttime behavior, cannot be completely ruled out, although several checks by the authors argued against this explanation. The heart scans were taken roughly three years after light exposure was measured, and structural heart changes typically take years to develop, so some uncertainty in timing remains. Finally, the study population was overwhelmingly White and generally healthier than the broader public, and shift workers were excluded, which may limit how well the findings apply to more diverse populations or people with irregular work schedules.
Funding and Disclosures
The study was supported by grants from the National Heart, Lung, and Blood Institute, the National Institute of Diabetes and Digestive and Kidney Diseases, the Fogarty International Centre, and Tulane Research Centres of Excellence Awards. The corresponding author also received support from additional National Institutes of Health funding. The authors reported no disclosures of interest related to this work.
Publication Details
The study, titled “Nighttime light exposure and cardiac structure and function,” was authored by Jin Dai, Wen Dai, Yoriko Heianza, and Lu Qi, affiliated with Tulane University’s School of Public Health and Tropical Medicine, the Louisiana Department of Health, and Harvard T.H. Chan School of Public Health. It was published in the European Heart Journal. DOI: 10.1093/eurheartj/ehag563.
