Sleeping with curtains open? Study reveals city night light may raise heart disease risk

City lights glowing up the room or bright lamps may affect the body's circadian rhythm, increasing the chances of coronary artery disease.

Do you have a habit of sleeping with your curtains open, letting the street lamp's light dimly glow up your room? This habit may seem calming, but you may want to think twice, as it puts your heart health at risk. A study published in June 2025 revealed an interesting, even surprising, risk factor for heart disease: night light.

Draw the curtains and sleep as the night lift seeping through window may affect your cardiac health. (Shutterstock)

Body's internal clock gets affected

The study highlighted the importance of a dark environment for a good night's sleep, which, in turn, maintains good heart health. Exposure to night light affects heart health adversely. This is because our bodies depend on natural light to sleep and wake up. Other than managing bedtime schedules, this internal clock, also known as the circadian rhythm, controls many other bodily functions too, from blood pressure to blood sugar levels. But this bodily rhythm is thrown off balance by artificial light. At night, there are many sources of artificial light, including street lamps, TVs, and smartphones.

To understand the biological reasoning further, the study pinpointed the mechanism of hypercoagulability, which is the increased likelihood of forming blood clots, with one of the causes being a disturbed circadian rhythm.

What does this mean?

Any kind of artificial light before bed, whether it is phone's light or street lamp, can disrupt the body's internal clock and in turn increase risks of heart diseases. (Shutterstock)

The researchers obtained data from 88,905 adults who wore wrist sensors to track light exposure over the course of a week. The follow-ups then spanned over a decade. Those people who were in the top 10 per cent, significantly exposed to light at night, developed higher risks of coronary artery disease, heart attack, heart failure, atrial fibrillation, and stroke. Other secondary lifestyle and genetic factors were also taken into consideration.

The study further reminded that this shows an association, a connection, not a direct cause-and-effect relationship. Particularly, women are at higher risk when it comes to heart failure and coronary artery disease, and for younger adults, other than heart failure, they also have risks of atrial fibrillation.

Some practical steps that benefit your heart health include sleeping with curtains drawn, avoiding phones before bed, and dimming or switching off lamps in the bedroom before sleep.

Note to readers: This article is for informational purposes only and not a substitute for professional medical advice. Always seek the advice of your doctor with any questions about a medical condition.