Imagine this: You’ve just settled into bed, the room bathed in the soft glow of a nightlight by your bedside—a small comfort to ease midnight trips to the bathroom or chase away the dark. It feels harmless, even warm and cozy. But what if that gentle beacon, meant to soothe, is secretly plotting against your heart? Recent science isn’t mincing words: the brighter the light while you sleep, the heavier the toll on your cardiovascular system. Your heart, it turns out, prefers the dark.
Many people rely on "cozy nightlights" for convenience or to combat fear of the dark, unaware that this habit may pose a significant threat to cardiovascular health. Emerging scientific evidence reveals a clear link: increased nighttime light exposure correlates with heightened heart strain and elevated risks of cardiovascular diseases.
A study by Flinders University, analyzing data from 88,905 participants in the UK Biobank, found that strong nighttime light significantly raises the risk of conditions like coronary heart disease, myocardial infarction, and heart failure—with the highest risk increasing by up to 56%.
The research highlights a dose-response relationship, where greater light exposure corresponds to higher risks, even after accounting for factors such as age, gender, lifestyle, and genetic susceptibility. Conversely, daytime light exposure shows protective effects for certain cardiovascular conditions, though this diminishes when adjusting for lifestyle factors. Another study published in Proceedings of the National Academy of Sciences (PNAS) further supports these findings, demonstrating that moderate nighttime light during sleep impairs cardiovascular function and increases insulin resistance, amplifying risks of heart disease and diabetes. These studies collectively underscore the critical need to minimize nighttime light exposure to safeguard heart health.
For some, a "cozy nightlight" is a nightly companion—whether to make late-night bathroom runs easier or to fend off the dark. Little do they know, this seemingly harmless habit could be one of the hidden culprits endangering cardiovascular health. This isn’t a joke: science confirms that the brighter the nighttime light, the greater the burden on the heart, and the more precarious heart health becomes.
Recently, researchers from Flinders University in Australia published their findings, investigating whether daytime and nighttime light exposure can predict the incidence of cardiovascular diseases, and whether the relationship between light exposure and cardiovascular diseases varies based on genetic susceptibility, gender, and age. The study revealed that exposure to strong nighttime light significantly boosts the risk of various cardiovascular conditions, including coronary heart disease, myocardial infarction, and heart failure—with the highest risk increase reaching a staggering 56%!
DOI: https://doi.org/10.1101/2025.06.20.25329961
The research team included 88,905 participants from the UK Biobank. These participants wore wrist-worn light sensors for a week to accurately record the intensity of light they were exposed to during both day and night. By linking data from the UK National Health Service (NHS) records, the team tracked new cases of cardiovascular diseases among participants up until the end of 2022. Their focus was on five key conditions: coronary artery disease, myocardial infarction (heart attack), heart failure, atrial fibrillation, and stroke. The analysis rigorously accounted for factors such as age, gender, ethnicity, seasonal daylight hours, socioeconomic status, lifestyle (smoking, alcohol consumption, diet, physical activity), sleep metrics, clinical risk factors (e.g., obesity, diabetes), and even genetic risk scores (polygenic risk).
Day-Night Light Differences Impact Cardiovascular Health: Nighttime Increases Risk, Daytime Offers Protection
With an average follow-up period of 7.9 ± 1.0 years, the analysis uncovered a significant association between nighttime light exposure and elevated cardiovascular disease risk, with a clear dose-response pattern. When compared to those in the lowest light exposure group (0-50th percentile), individuals exposed to stronger nighttime light faced escalating risks of coronary heart disease as exposure levels rose: 1.11-1.12 times higher for the 50-70th percentile, 1.18-1.20 times for the 70-90th percentile, and 1.23-1.32 times for the 90-100th percentile.
Similarly, risks of myocardial infarction and heart failure climbed in a graded manner, with the highest exposure group (90-100th percentile) seeing increases of 1.42-1.47 times and 1.45-1.56 times, respectively. The 90-100th percentile group also showed significantly higher risks of atrial fibrillation and stroke, ranging from 1.28-1.32 times. Even after adjusting for diabetes, hypertension, high BMI, high cholesterol, sleep abnormalities, and excluding shift workers, nighttime light remained a consistent predictor of cardiovascular risk.
In contrast, strong daytime light exposure demonstrated protective effects against certain cardiovascular diseases. In models 1-2, the highest exposure group (90-100th percentile) showed a 13% reduction in coronary heart disease risk, a 28% reduction in heart failure risk, and a 27% reduction in stroke risk. However, the downward trends for myocardial infarction and atrial fibrillation did not reach statistical significance, and the protective effects of daytime light vanished when adjusting for lifestyle factors.
Nighttime Light and Cardiovascular Risk: Pronounced Differences by Gender and Age, No Role for Genetic Susceptibility
The study found that the impact of nighttime light on cardiovascular disease risk varies with age and gender. Men and older individuals face higher overall risks for all cardiovascular diseases. The strength of the relationship between nighttime light and risks of heart failure, coronary heart disease, and atrial fibrillation differs notably across groups: women and younger individuals have a higher risk of heart failure; women face a greater risk of coronary heart disease; younger individuals are more susceptible to atrial fibrillation; and the link between light exposure and myocardial infarction risk is unaffected by age or gender.
Furthermore, the association between nighttime light and cardiovascular risk remained robust even after adjusting for polygenic risk scores for cardiovascular disease. However, no significant interaction was found between light exposure and polygenic risk, indicating that genetic susceptibility does not alter the effect of light on heart failure, coronary heart disease, or atrial fibrillation.
Taken together, these findings make it clear: the brighter the nighttime light, the louder the "protest" from your heart and blood vessels—with risks of heart failure, coronary heart disease, atrial fibrillation, and more all skyrocketing.
Adding to this body of evidence, another study published in Proceedings of the National Academy of Sciences (PNAS) found that exposure to moderate ambient light during nighttime sleep impairs cardiovascular function during sleep and increases insulin resistance the following morning, thereby raising the risk of heart disease and diabetes.
This study included 20 healthy adults, randomly assigned to either a dim light group or a moderate light group. The analysis revealed significant differences in changes to the Homeostatic Model Assessment for Insulin Resistance (HOMA-IR) between the two groups from day 1 to day 2. Participants who slept under moderate light conditions saw their HOMA-IR increase by approximately 15%, while those in dim light experienced a roughly 4% decrease. Additionally, differences in insulin levels and insulin sensitivity index changes between day 1 and day 2 were more pronounced in the moderate light group compared to the dim light group.
The study also noted that, from day 1 to day 2, participants in the moderate light group experienced a significant increase in heart rate (HR). Changes in the low-frequency/high-frequency (LF/HF) ratio—an indicator of cardiac autonomic function—were significantly greater in the moderate light group than in the dim light group.
In summary, this research demonstrates that even a single night of sleep under moderate light can impair cardiovascular regulation and boost insulin resistance—both of which are risk factors for heart disease, diabetes, and metabolic syndrome.
For those in Europe and America, where nightlights, streetlights seeping through windows, or even the glow of electronic devices are common fixtures of nighttime life, these findings hit close to home. The science is clear: that seemingly innocent nightlight or the faint glow from your phone on the nightstand might be doing more harm than good.
So, as you prepare for bed tonight, take a moment to assess your bedroom. Flip off unnecessary lights, draw those blackout curtains tight, and let your heart enjoy the rest it deserves—in the dark. Your cardiovascular system will thank you.