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In A Nutshell
Sleeping with the lights on is linked to a 42% higher risk of heart attack compared to sleeping in darkness, based on a study tracking nearly 90,000 adults over 9 years.
The brightest bedrooms also showed higher risks for heart failure, coronary artery disease, irregular heartbeat, and stroke.
The connection held even after accounting for diet, exercise, smoking, and genetics, suggesting light at night is a factor worth managing for heart health.
Simple fixes like blackout curtains, turning off devices, and keeping bedrooms dark may help protect your heart.
Maybe it’s the glow from your alarm clock, the TV you fall asleep to, or the hallway light you leave on throughout the night. Whatever the source, sleeping with ambient light might be doing more than disrupting your sleep—it could be affecting your heart. A study of 89,000 people found those with the brightest bedrooms had a 42% higher rate of heart attacks over nearly 10 years.
The connection held true regardless of how much people exercised, what they ate, whether they smoked, or how their genes predisposed them to heart disease. Light exposure at night remained linked to cardiovascular health even after accounting for familiar risk factors like poor diet and lack of physical activity.
The study also found elevated risks for heart failure, coronary artery disease, irregular heartbeat, and stroke. Researchers tracked 88,905 people in the United Kingdom who wore light-tracking devices on their wrists for one week, then followed their health records for the next 9.5 years. The average age was 62, and about 57% were women.
How Researchers Measured Personal Light Exposure Around the Clock
Participants wore small sensors on their wrists that recorded light levels around the clock for seven days, collecting approximately 13 million hours of light data total. Rather than relying on satellite data measuring outdoor lighting, this approach captured personal exposure.
Researchers identified daytime light exposure (7:30 a.m. to 8:30 p.m.) and nighttime light exposure (12:30 a.m. to 6:00 a.m.), then grouped participants by their light levels. Those with the darkest nights (the bottom half) served as the comparison group.
People in moderately bright rooms had a 20% higher risk of heart attack. Those in brighter rooms faced a 27% higher risk. The brightest rooms showed a 47% increase. After accounting for age, sex, ethnicity, season, socioeconomic status, physical activity, smoking, alcohol consumption, and diet quality, the risk for the brightest rooms remained at 42%.
Bright bedrooms impact heart health as well as sleep quality. (Credit: Jozeph Forakis via SWNS)
Why Light at Night Disrupts Your Heart Health
The human body runs on circadian rhythms (internal clocks that regulate everything from blood pressure to heart rate to the release of stress hormones). Light is the primary signal that keeps these rhythms synchronized with the 24-hour day.
When light hits the eyes at night, it disrupts these rhythms. Blood pressure normally dips during sleep, but exposure to light can keep it elevated. Hormones that should peak in the morning may be released at the wrong times. The balance between the sympathetic nervous system (which activates “fight or flight” responses) and the parasympathetic system (which promotes “rest and digest”) gets thrown off.
Over the short term, circadian disruption raises heart rate, increases inflammation, and promotes blood clotting. Over years, these changes can contribute to atherosclerosis (the hardening of the arteries that sets the stage for heart attacks). When arterial plaques rupture or blood clots form in narrowed vessels, heart muscle tissue gets starved of oxygen, causing a heart attack.
Research on shift workers, who regularly experience bright light during nighttime hours, has documented higher rates of heart disease for decades. This new study extends those findings to the general population, showing that even ambient light in bedrooms or from devices can carry risks.
The study, published in JAMA Network Open, also found that circadian disruption may increase the risk of irregular heartbeats. When the body’s master clock in the brain sends conflicting signals to the heart’s own cellular clocks, the result can be irregular heartbeats that increase stroke risk and reduce the heart’s pumping efficiency.
Who Is Most Affected? Women and Younger Adults at Higher Risk
Women appeared more vulnerable to the heart-damaging effects of nighttime light than men, particularly for coronary artery disease and heart failure. Previous research has found that women may be more sensitive to bright light than men, with their circadian systems responding more strongly to the same light levels.
Younger adults in the study also showed larger increases in risk for heart failure and atrial fibrillation compared to older participants. This pattern could reflect that older adults generally have less sensitive circadian systems, meaning their bodies may not respond as strongly to nighttime light exposure.
Genetic susceptibility to heart disease didn’t change the relationship between light and cardiovascular risk. People with higher genetic risk scores still showed the same association between bright nights and increased disease as those with lower genetic risk.
The Surprising Benefits of Daytime Light Exposure
At first glance, brighter daytime light appeared protective. People exposed to the brightest daytime light had a 13% lower risk of coronary artery disease, a 28% lower risk of heart failure, and a 27% lower risk of stroke compared to those with the darkest days. However, these protective effects disappeared after adjusting for lifestyle factors, particularly physical activity. People who spend time outdoors in bright daylight are also more likely to be physically active, and exercise is a well-established protective factor for heart health.
When researchers looked at the effect of bright days without factoring in exercise, the protective effect reappeared for heart failure and stroke, suggesting that daytime light exposure and physical activity may work together to support cardiovascular health.
What This Means for Protecting Your Heart Health
The research was limited by a number of factors warranting acknowledgment. Light exposure was measured for only one week, though participants’ light patterns remained fairly consistent over time. The study couldn’t determine specific light sources (streetlights, bedroom lamps, televisions, or smartphones). The UK Biobank participants are predominantly White (97%), better educated, higher income, and healthier than the general population. The study is also observational, meaning it shows associations but can’t prove direct causation.
Cardiovascular diseases remain the leading cause of death worldwide. This study points to avoiding light at night as a new prevention tool. Unlike genetic risk factors, light exposure is modifiable through blackout curtains, turning off devices, using dim red lights for necessary nighttime activities, and keeping bedrooms dark.
For shift workers or those with unavoidable light exposure, managing other cardiovascular risk factors through diet, exercise, and regular health screenings becomes especially important. Darkness matters for heart health. This research shows that sleeping in darkness appears important beyond just sleep duration.
Disclaimer: This article summarizes scientific research for general informational purposes only. It is not intended as medical advice. If you have concerns about your heart health or sleep environment, please consult with a qualified healthcare professional.
Paper Summary
Methodology
Researchers analyzed data from 88,905 UK Biobank participants who wore light-tracking sensors on their wrists for one week between 2013 and 2016. The sensors recorded light exposure continuously, capturing approximately 13 million hours of data. Participants were then followed through health records until November 2022, a span of about 9.5 years. The researchers used factor analysis to identify two distinct time windows: daytime light exposure (7:30 a.m. to 8:30 p.m.) and nighttime light exposure (12:30 a.m. to 6:00 a.m.). They divided participants into four groups based on their light exposure percentiles: 0-50th (darkest), 51st-70th, 71st-90th, and 91st-100th (brightest). Cardiovascular disease diagnoses came from hospital admission records, primary care records, self-reports, and death registers. The conditions tracked included coronary artery disease, myocardial infarction (heart attack), heart failure, atrial fibrillation, and stroke. Statistical models used Cox proportional hazards analysis adjusted at three levels for various confounding factors.
Results
Participants exposed to the brightest nighttime light (91st-100th percentiles) had significantly higher risks for all five cardiovascular outcomes compared to those in the darkest environments (0-50th percentiles). The adjusted hazard ratios in the most adjusted model were 1.23 for coronary artery disease, 1.42 for myocardial infarction, 1.45 for heart failure, 1.28 for atrial fibrillation, and 1.28 for stroke. Dose-response relationships were observed, meaning higher light exposure correlated with higher disease risk across the percentile groups. The associations remained statistically significant after adjusting for established cardiovascular risk factors including physical activity, diet, smoking, alcohol consumption, sleep duration, pre-existing diabetes, hypertension, body mass index, cholesterol levels, and genetic susceptibility. Women showed larger-magnitude associations between nighttime light and heart failure and coronary artery disease compared to men. Younger participants in the cohort showed stronger associations between nighttime light and heart failure and atrial fibrillation compared to older participants. Brighter daytime light showed some protective associations with cardiovascular outcomes in initial models, but these effects were not statistically significant after adjusting for lifestyle factors, particularly physical activity.
Limitations
Light exposure was measured for only one week per participant, which may not fully capture long-term patterns, although previous research indicates individual light patterns remain relatively stable over time. The study could not determine the sources of nighttime light exposure, so researchers couldn’t distinguish between different types of light (streetlights, electronic devices, indoor lighting). The UK Biobank cohort is predominantly White (97%), better educated, higher income, and healthier than the general UK population, which may limit how well the findings apply to more diverse groups. Some covariates such as physical activity and pre-existing health conditions may lie on the causal pathway between light exposure and cardiovascular disease, making it difficult to determine whether adjusting for them is appropriate. The study is observational, meaning it can show associations but cannot prove that nighttime light directly causes cardiovascular disease. Information about some variables was collected before light tracking and may have changed during the follow-up period.
Funding and Disclosures
This work was supported by the Australian Research Council (grant numbers DP210102924 and DP220102812 to Dr. Phillips) and the National Institutes of Health (grant numbers R35-GM146839 and R01-HG012810 to Dr. Lane and grant numbers R01-HL140574, R01-HL153969, R01-HL164454, and R01-HL167746 to Dr. Scheer). Dr. Rutter reported receiving personal fees from Eli Lilly for consultancy and having stock ownership in GSK outside the submitted work. Dr. Scheer reported serving on the Board of Directors for the Sleep Research Society and receiving consulting fees from the University of Alabama at Birmingham, Morehouse School of Medicine, and Salk Institute for Biological Studies outside the submitted work. Dr. Cain reported being a cofounder of Circadian Health Innovations, having a pending patent without royalties, receiving research funding from Versalux and Delos, having consulted for Dyson, Colorbeam, and Beacon Lighting, and receiving a philanthropic donation from Beacon Lighting outside the submitted work. Dr. Phillips reported receiving grants from Beacon and Versalux and being a cofounder of Circadian Health Innovations outside the submitted work.
Publication Details
Windred DP, Burns AC, Rutter MK, Lane JM, Saxena R, Scheer FAJL, Cain SW, Phillips AJK. “Light Exposure at Night and Cardiovascular Disease Incidence,” JAMA Network Open. 2025;8(10):e2539031. doi:10.1001/jamanetworkopen.2025.39031. Published October 23, 2025.