Exposure to even low levels of light during sleep may increase the risk of heart disease, according to a study published in the European Heart Journal. Researchers found that individuals who slept with more than three lux of light—roughly equivalent to bright moonlight—showed thickening of their left ventricle walls.
The left ventricle is vital for pumping blood throughout the body. Thickened walls reduce the chamber's capacity, forcing the heart to work harder and potentially leading to heart attack, rhythm disorders, and stroke. The study also indicated that these individuals experienced reduced flexibility in their heart muscles during beats, a sign of "cardiac remodeling," or structural changes that impair the heart's function.
Researchers from Tulane University in New Orleans analyzed sleep habits and light exposure data from over 11,000 participants aged 40 to 69, who were recruited from the UK Biobank. For one week, participants wore wrist sensors to measure ambient light levels during sleep. Three years later, heart MRI scans were conducted to assess cardiac structure and function.
Of the participants, about 25% had no nighttime light exposure above three lux. The scans revealed that those exposed to at least three lux at night had larger cardiac chamber volumes in their left and right ventricles, as well as the left atrium. They also exhibited greater wall mass and thickness in the left ventricle, along with increased strain in this area.
The study authors estimate that between 24% and 49% of the heart's reduced function was attributed to shorter sleep durations, averaging five to six hours, which they suggest is a direct consequence of nighttime light exposure. "Light pollution has emerged as a new risk factor for cardiovascular disease," said Dr. Lu Qi, a senior study author and director of the Tulane University Obesity Research Center. "Our findings, together with evidence from other studies, suggest that reduction of nighttime light exposure should be considered as one of the potential strategies for preventing heart disease by clinicians and policymakers."
Common sources of light pollution include the glow from smartphones, digital alarm clocks, and external lights that seep into a room. The researchers recommend minimizing light in the bedroom, aiming for no more than one lux—a level comparable to a very dim room lit by a single candle several feet away. This can be achieved using blackout curtains and turning off unnecessary lights.
Heart disease remains the leading cause of death in the United States, with nearly a million Americans dying from it each year. The study acknowledges limitations, including measuring light exposure for only one week and using wrist sensors rather than direct eye measurement. However, the findings contribute to a growing body of evidence suggesting that controlling light exposure during sleep is an important factor for cardiovascular health.