Key points from article :
Exposure to light at night may significantly raise the risk of heart disease by disrupting the body’s internal clock, according to a large new study led by Angus Burns, a postdoctoral fellow at Harvard Medical School and Brigham and Women’s Hospital. The research, co–first authored by Daniel Windred of Flinders University in Australia, was published in JAMA Network Open in October.
Using data from nearly 89,000 participants in the UK Biobank, the researchers tracked nighttime light exposure with wrist-worn devices and followed participants’ health outcomes for an average of nine and a half years. They found that people exposed to the brightest levels of light during the night had a 30–50% higher risk of cardiovascular problems, including heart attack, stroke, heart failure, atrial fibrillation, and coronary artery disease, compared with those who slept in the darkest conditions.
Crucially, the increased risk was not explained by poor sleep alone. Instead, the findings point to disruption of the circadian rhythm—the body’s master biological clock—which regulates not just sleep, but the timing and function of nearly every organ. The body is especially sensitive to light between midnight and 6 a.m., when exposure can effectively “reset” internal clocks and send mistimed signals throughout the body.
The study showed a clear dose–response relationship: even moderate nighttime light exposure raised cardiovascular risk, with the danger increasing steadily as light levels went up. These effects were independent of traditional risk factors such as smoking, diet, physical activity, alcohol use, and sleep duration, suggesting that light exposure itself is a distinct and modifiable risk factor.
The researchers argue that modern lifestyles may be particularly hostile to circadian health. Many people experience nights that are too bright and days that are too dim, spending most of their time indoors under weak artificial lighting. Together, these patterns may weaken the signals that keep the body’s clocks aligned—leaving us groggy by day, restless at night, and more vulnerable to long-term disease.
