A study published September 9 in the European Heart Journal found that exposure to light while sleeping, even at low levels, is linked to structural changes in the heart and a higher risk of cardiovascular disease.
Researchers analyzed data from 11,071 adults in the UK Biobank who had no cardiovascular disease at the start of the study. The average participant age was about 61, and 56.6% were women. Participants wore a wrist device with a light sensor for seven consecutive days between 2013 and 2015 to measure actual nighttime light exposure. Three years later, researchers performed MRI scans to check for changes in heart structure.
People with the highest nighttime light exposure showed measurable differences: 2.4% greater left ventricular mass, 1.5% greater average heart wall thickness, and a 1.9% lower myocardial contraction fraction, a measure of how effectively the heart muscle contracts.
After adjusting for age, sex, activity level, diet, obesity, and other health conditions, researchers found that exposure above three lux was associated with a 29% higher risk of heart failure, a 15% higher risk of atrial fibrillation, a 24% higher risk of heart attack, a 33% higher risk of stroke, and a 26% higher risk of dying from cardiovascular disease.
“Light pollution has emerged as a new risk factor for cardiovascular disease,” said senior study author Dr. Lu Qi. “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 policy makers.”
The researchers concluded that greater nighttime light exposure was associated with adverse cardiac remodeling and subclinical functional decline across multiple heart chambers, a relationship statistically mediated by shorter sleep duration.
What counts as too much light?
The three-lux threshold used in the study is not total darkness, but a dim level where a person can barely make out their surroundings. Measuring lux in a real bedroom is not straightforward, since distance, brightness settings, direction, room size, and window coverings all affect how much light actually reaches the sleeper.
A night light can exceed three lux if it is bright or positioned close to the bed, while a fainter one across the room might not. A bedside lamp, when on, would typically be well above that level, and one laboratory study used 50 lux to represent typical bedroom-intensity light. Light from a hallway or bathroom seeping under a door, or streetlights and porch lights coming through a window, can also push exposure above the threshold depending on brightness and distance.
Phones and tablets present a separate concern. A controlled study found that evening use of such devices suppressed melatonin, the hormone that helps regulate sleep, by about 55% compared with reading a printed book.
The study is observational, meaning it shows an association rather than proof that light exposure directly causes heart damage. Researchers did not recommend eliminating every source of ambient light, but the findings suggest that darker bedrooms and adequate sleep duration may be worth considering as part of efforts to protect cardiovascular health.
