Is Sleeping With a Light On Bad for Your Heart? What a Major 2026 Study Found
Updated: 2 hours ago
Written and medically reviewed by Dr. Baraa Alnahhal, MD · Last reviewed: September 2026
Nighttime light exposure is the light that reaches you while you are asleep — a hallway lamp, a streetlight through thin curtains, a television left running. A major cohort study published on 9 September 2026 in the European Heart Journal found that adults exposed to more of this light had thicker, less flexible, weaker-pumping heart muscle on MRI scans three years later, and a higher rate of heart failure, stroke and heart attack over the following decade. If you have wondered whether sleeping with a light on is bad for your heart, here is what the research shows, and what it does not.
Key takeaways
Researchers tracked 11,071 UK Biobank adults who wore a light-sensing wristband for a week, then had a detailed cardiac MRI about three years later.
People with high nighttime light exposure (above 3 lux) had 2.4% more left ventricular muscle mass, 1.5% thicker heart walls, and about 1.9% weaker contraction than people with none.
In a larger group of 73,286 people followed for 8–10 years, high nighttime light exposure was associated with a 29% higher risk of heart failure and a 33% higher risk of stroke.
Roughly 24–49% of the effect was explained by simply sleeping less — the rest was not.
This is an observational study. It shows a strong, dose-dependent association, not proof that light causes heart damage.
What did the new European Heart Journal study actually find?
Researchers at Tulane University and the Harvard T.H. Chan School of Public Health measured how much light reached 11,071 UK adults while they slept, using wrist-worn sensors over seven nights. Three years later, those participants had cardiac MRI scans. People exposed to more than 3 lux at night showed a pattern doctors call concentric hypertrophy: a heavier, thicker-walled left ventricle that squeezes less efficiently (Dai et al., European Heart Journal, 2026).
How big were the changes to the heart?
Small on paper, consistent across every chamber measured. Compared with people who had no measurable light at night, the high-exposure group showed:
Heart measurement | Change vs. no nighttime light | 95% confidence interval |
|---|---|---|
Left ventricular mass (indexed) | +2.4% | 1.6% to 3.1% |
Mean wall thickness | +1.5% | 1.0% to 2.0% |
Myocardial contraction fraction | −1.9% | 1.1% to 2.7% |
Global circumferential strain | −2.7% | −3.7% to −1.7% |
Global longitudinal strain | −2.8% | −3.9% to −1.7% |
Left atrial maximum volume (indexed) | +2.3% | 0.7% to 3.9% |
Left atrial ejection fraction | −1.1% | 0.3% to 1.9% |
Strain is a measure of how much the heart muscle deforms as it beats — lower absolute strain means a stiffer, less springy muscle. Every association followed a linear dose-response pattern: more light, more change. That consistency is one of the reasons the finding is being taken seriously.
Does nighttime light actually raise the risk of heart disease?
In this dataset, more light was associated with more disease. Across 73,286 participants followed for 8–10 years, those with high nighttime light exposure had a 29% higher risk of heart failure, 33% higher risk of stroke, 24% higher risk of heart attack, 15% higher risk of atrial fibrillation, and 26% higher risk of dying from cardiovascular disease than those with none.
Those are relative increases, not absolute ones. If your baseline ten-year risk of heart failure is low, a 29% relative increase on top of it is still a modest absolute change. The finding is meaningful at a population level; for any one person it is one risk factor among many.
Why would sleeping with a light on affect the heart?
Two overlapping mechanisms, and the study tested one of them directly. The researchers found that 24% to 49% of the association ran through shorter sleep duration — light kept people awake, and short sleep is an established cardiovascular risk factor. But more than half the effect did not travel that route, pointing to the circadian system itself.
Light striking the eye at night suppresses melatonin and shifts the body's internal clock. As Cleveland Clinic sleep disorders specialist Christen Cheuvront puts it, "Light exposure essentially causes a trickle-down effect that influences the timing of these bodily functions" — including overnight blood pressure dipping and heart rate, both of which normally fall during sleep and both of which matter for long-term cardiac load.
How dark does a bedroom need to be?
The threshold in this study was 3 lux — dim by everyday standards. For reference, standard illuminance charts put a clear full moon at roughly 0.1–0.3 lux and ordinary indoor room lighting somewhere between about 100 and 500 lux depending on the room; a dim plug-in night light sits in the low single digits. In other words, the "high exposure" group in this study was not sleeping under a floodlight. A bright alarm clock, a bathroom light left on, or an uncovered window facing a street lamp can reach this range.
What can you do tonight?
Johns Hopkins Medicine neurologist and sleep specialist Rachel E. Salas advises seven to nine hours of quality sleep and notes that "Sleep environment is a huge factor in getting good sleep". Practical steps, in rough order of impact:
Block outside light. Blackout curtains or blinds address the source most people can't switch off.
Wear a sleep mask if blackout coverings aren't practical — cheap, and it works at the eye, which is where light matters.
Turn off or cover the television. A screen left running is both light and noise.
Dim or turn away the alarm clock, and cover standby LEDs on chargers, routers and air purifiers.
Swap a hallway or bathroom night light for a dim, warm-toned, floor-level one if you need it for safe navigation — the goal is to reduce light at eye level, not to create a trip hazard.
Protect sleep length as well as darkness. Since up to half the association here ran through shorter sleep, a dark room that you only spend five hours in misses much of the point.
How much should you trust this study?
It is strong for its type and still limited. Strengths: large sample, objective light measurement rather than self-report, objective MRI outcomes, a separate and much larger outcomes analysis, and a clean dose-response curve.
Limitations matter just as much. This is observational, so it cannot establish that light causes the heart changes. Light was recorded for only seven nights, three years before the scans. Wrist sensors measure light at the wrist, which is an imperfect stand-in for light reaching the eye. And UK Biobank volunteers are, on average, healthier and less diverse than the wider population, so the numbers may not transfer exactly to everyone. People who sleep with lights on may also differ in other ways the researchers could not fully measure — shift work, noise, urban density, alcohol.
Frequently asked questions
Is it bad to sleep with a light on?
This study links higher nighttime light exposure to unfavourable changes in heart structure and a higher rate of cardiovascular disease. It does not prove light caused those changes. Sleeping in a dark room is a low-cost, low-risk habit with reasonable supporting evidence, so it is a sensible default.
Does a small night light count?
Possibly. The study's threshold was 3 lux, which is within reach of a typical plug-in night light at close range. If you need one for safety, a dim, warm-coloured light placed low and away from the bed is a reasonable compromise.
Does it matter what colour the light is?
This particular study measured overall light intensity, not colour, so it cannot answer that. Separate sleep research suggests blue-enriched light suppresses melatonin more strongly than warm light, which is why warm-toned night lights are generally preferred.
Should I change my heart medication because of this?
No. This is a single observational study about an environmental exposure. It is not a reason to alter any prescription. Discuss any medication question with your own doctor.
Is blackout curtain evidence or marketing?
The evidence here supports darkness, not any particular product. A sleep mask, taped-up LEDs and a closed door achieve the same goal for far less money.
Medical disclaimer
This article is health information, not personal medical advice. It summarises published research and does not account for your individual history, medications or risk factors. Always speak with a qualified healthcare professional about your own health, and never start, stop or change a treatment based on a news article.
Primary sources cited
Dai J, Dai W, Heianza Y, Qi L. Nighttime light exposure and cardiac structure and function. European Heart Journal. 9 September 2026. DOI: 10.1093/eurheartj/ehag563 · PubMed
Cleveland Clinic. Is It Bad To Sleep With the Lights On?
Johns Hopkins Medicine. Five Ways to Sleep Well and Protect Your Heart
About the author: Dr. Baraa Alnahhal, MD, is a medical doctor and the founder of Rinnit. Dr. Baraa Alnahhal reviews new medical research and translates it into clear, evidence-based health information for everyday readers.
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