Researchers find that even relatively low levels of light exposure at night may be associated with changes in the heart’s structure and function
A hallway light left on outside a bedroom. A phone glowing on the bedside table. A television playing quietly in the background.
These may seem like harmless features of a modern bedroom, but new research suggests that exposure to light while sleeping could be associated with changes in the heart.
Researchers at Tulane University’s Celia Scott Weatherhead School of Public Health and Tropical Medicine have found that people exposed to higher levels of light at night showed structural and functional changes in their hearts several years later.
The study, published in the European Heart Journal, examined more than 11,000 participants and used wearable light sensors to measure how much light they were exposed to overnight.
The findings add to growing scientific interest in the potential relationship between nighttime light exposure and cardiovascular health.
“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,” says lead author Lu Qi, a chair and professor at Tulane.
“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.”
Measuring darkness
The research team analysed data from 11,017 participants who wore a sensor on their wrists for seven days.
The devices measured the amount of light participants encountered during the night, allowing researchers to examine differences between people sleeping in near darkness and those exposed to greater amounts of artificial light.
Some participants experienced approximately 0 lux, representing near-total darkness.
Others experienced progressively higher levels of exposure, with the highest-exposure group receiving more than 3 lux.
To put that figure into everyday terms, researchers describe it as roughly comparable to the light from a phone screen at low brightness on a bedside table or a hallway light shining through a partially open bedroom door.
The participants were then followed over time.
Approximately three years later, they underwent cardiovascular magnetic resonance imaging, or CMR, which allowed researchers to examine the structure and function of their hearts in greater detail.
What the scans revealed
The researchers found that participants exposed to the highest levels of nighttime light had changes in the structure of the heart's left ventricle, the chamber responsible for pumping oxygen-rich blood around the body.
The walls of the left ventricle were thicker, leaving less space within the chamber.
The researchers also observed indications that the heart had become less flexible during the heartbeat.
Such changes are important because alterations in the heart's structure and ability to function can be associated with cardiovascular problems.
“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,” Qi says.
“This is where the structure and function of the heart change, 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.”
Why nighttime light is attracting attention
For much of human history, darkness was a natural part of nighttime.
Modern life has changed that.
Electricity, smartphones, televisions, computers, streetlights and illuminated buildings mean that many people now spend their nights surrounded by artificial light.
Scientists have increasingly investigated whether this constant exposure could affect human health.
Nighttime light is already being studied as a potential environmental risk factor for cardiovascular disease. The new research adds another dimension by examining not only disease risk but also possible changes to the physical structure and functioning of the heart.
Cardiovascular disease remains the leading cause of death globally and includes conditions such as heart disease and stroke.
That makes potentially modifiable environmental factors particularly relevant to researchers.
Unlike factors such as age and genetics, nighttime light exposure can often be reduced through relatively simple changes to the sleeping environment.
Small changes, potentially meaningful habits
The researchers suggest that reducing exposure to artificial light during sleep could be one practical step people take to support cardiovascular health.
That could mean switching off unnecessary lights before going to bed, keeping smartphones and other bright screens away from the bedside, closing curtains or blinds to block outdoor lighting, or using an eye mask when a dark sleeping environment is difficult to achieve.
However, the study should not be interpreted as proof that nighttime light directly causes heart disease.
The researchers observed an association between higher nighttime light exposure and later cardiac changes. Observational findings can identify important relationships, but they do not by themselves establish that the light exposure was the direct cause of the changes.
Further research will be needed to understand the mechanisms involved and determine whether reducing nighttime light exposure can actually prevent or reverse cardiovascular changes.
Still, the findings contribute to a growing body of evidence suggesting that the environment in which people sleep may matter more than previously recognised.
“Light pollution has emerged as a new risk factor for cardiovascular disease,” Qi says.
“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.”
The bedroom as part of the health equation
The implications of the research are striking partly because nighttime light can be difficult to notice.
A faint glow from a phone. A streetlight filtering through curtains. A television left on after falling asleep.
None may seem significant on its own.
But the emerging research suggests that the conditions surrounding sleep could have consequences beyond simply determining how well a person rests.
For people looking for simple ways to create a darker sleeping environment, the starting point may be straightforward: switch off unnecessary lights, put screens away before bed and keep the bedroom as dark as practical.
The research does not mean that every source of nighttime light is dangerous, nor does it suggest that people should fear ordinary household lighting.
Instead, it highlights a question scientists are increasingly asking: when the body is supposed to be resting in darkness, what happens when the darkness never quite arrives?
For now, the answer appears to extend beyond sleep — and possibly to the heart.