Experts have long recommended a dark, light-free bedroom for better sleep. New research from Tulane University suggests that sleeping in near-total darkness may carry an added benefit: protecting the heart.

In a study published in the European Heart Journal, researchers at Tulane’s Celia Scott Weatherhead School of Public Health and Tropical Medicine found that even small amounts of nighttime light exposure were linked to potentially harmful changes in the heart’s structure and function. The research team analyzed data from 11,017 participants who each wore a wrist sensor for a week to measure overnight light exposure. Some participants experienced 0 lux, or near-total darkness, while others were exposed to varying levels of light, with the highest group exposed to more than 3 lux — roughly equivalent to a phone screen at low brightness on a nightstand, or a hallway light left on outside a cracked door.

Measurable Changes Three Years Later

Three years after the initial measurement, each participant underwent a cardiovascular magnetic resonance scan to examine any changes in heart structure and function. Participants exposed to the highest levels of nighttime light showed thickening in the walls of the heart’s left chamber, reducing the space inside the ventricle. There were also signs that the heart’s ability to flex during a heartbeat had diminished — an early indicator of heart dysfunction.

“Previous observational studies have linked nighttime light exposure with a higher risk of cardiovascular diseases, but little is known about the related cardiac structure and functional changes. We carried out this research to find out what happens to the heart over time when people are exposed to too much light at nighttime,” said lead author Lu Qi.

“This is the first study of its kind, and it shows that exposure to higher levels of light at nighttime is linked to cardiac remodeling. This is where the structure and function of the heart changes, and we typically see it in response to chronic stress or injury to the heart. It’s our body’s way of adapting to that stress, but ultimately it weakens the heart and can lead to heart failure,” Qi said.

A Risk Factor Within Your Control

Cardiovascular disease, including heart disease and stroke, remains the leading cause of death both in the United States and worldwide. Nighttime light has been identified as a potential environmental risk factor for this deadly group of diseases — but unlike age or genetics, it’s a modifiable one, addressable simply by turning off screens, drawing curtains before bed, or sleeping with an eye mask.

“Light pollution has emerged as a new risk factor for cardiovascular disease. Our findings, together with evidence from other studies, suggest that a reduction in nighttime light exposure should be considered as one of the potential strategies for preventing heart disease by clinicians and policymakers,” Qi said.

The study, “Nighttime light exposure and cardiac structure and function,” was published in the European Heart Journal (DOI: 10.1093/eurheartj/ehag563) and was funded by the National Heart, Lung and Blood Institute, the National Institute of Diabetes and Digestive and Kidney Diseases, and the Fogarty International Center, all part of the NIH.