- New research shows that even dim light during sleep may thicken heart muscle walls and reduce heart flexibility, potentially raising cardiovascular risk.
- The findings build on prior evidence linking nighttime light exposure to cardiovascular disease risk.
- Optimizing your sleep environment can help protect your heart health.
Sleeping in dark conditions may be important not only for quality sleep but also for heart health
New research suggests that nighttime light exposure is associated with subtle changes in the heart, and even low levels of light — as low as 3 lux — could have negative effects over time
For context, moonlight is less than 3 lux, whereas the glow of a smartphone is much higher
Researchers analyzed data from more than 11,000 UK Biobank participants who wore wrist devices measuring light exposure. At around a three-year follow-up, the researchers used cardiovascular magnetic resonance imaging (MRI) to examine their hearts
Participants exposed to higher levels of nighttime light showed signs of cardiac remodeling, including slightly thicker heart muscle and subtle changes in heart contraction. These changes, the researchers noted, could lead to heart failure or atrial fibrillation (AFib)
Prior observational studies have linked nighttime light exposure to a higher risk of cardiovascular disease, but the effects on the heart’s structure and function were less understood
The new study builds on evidence connecting nighttime light exposure and cardiovascular disease, but cannot prove that light itself causes these changes
The findings were published in the European Heart Journal on September 9
“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,” lead study author Lu Qi, MD, PhD, a professor in the Epidemiology Department at Tulane University and director of the Tulane University Obesity Research Center, said in a news release
“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 continued
Low-level light exposure during sleep linked to heart changes
Researchers focused on nighttime light exposure above 3 lux, a relatively low level of illumination
This threshold was used to distinguish participants who experienced measurable nighttime exposure from those who did not
Compared with participants with no nighttime exposure above 3 lux, those with higher exposure had a 2.4% greater left ventricular mass, a 1.5% greater average heart wall thickness, and a 1.9% lower myocardial contraction fraction
The researchers also observed impaired myocardial deformation and subtle changes involving the right ventricle and left atrium
These differences were small and did not indicate cardiovascular disease
However, they may offer clues about how nighttime light exposure could contribute to cardiovascular problems over the long term
According to Qi, these heart changes could lead to the development of atrial fibrillation (AFib) or heart failure
“Our data indicate short sleeping may account for a high proportion of the observed adverse association, suggesting nighttime exposure may affect sleep duration and quality, which subsequently increases CVD risk,” Qi told Healthline
Cheng-Han Chen, MD, a board certified interventional cardiologist and medical director of the Structural Heart Program at MemorialCare Saddleback Medical Center in Laguna Hills, CA, said that while the exact mechanisms of this association are not fully understood, several factors may be involved. Chen wasn’t part of the study
“Previous research has linked nighttime light exposure with cardiovascular risk factors such as high blood pressure, high cholesterol
, obesity
, and diabetes,” Chen told Healthline
“Over time, these conditions can contribute to changes in the heart’s structure and function and increase cardiovascular risk. Nighttime light may also disrupt
the body’s natural circadian rhythm, which plays an important role in regulating blood pressure, metabolism, sleep, and healthy heart function.”
Nighttime light exposure and cardiovascular disease risk
The MRI findings in the present study are consistent with a 2025 JAMA Network Open
study involving nearly 89,000 UK Biobank participants
Researchers measured personal light exposure for one week and followed cardiovascular outcomes for about 9.5 years
Compared with people exposed to the darkest nighttime conditions, those in the brightest-light group had higher risks of coronary artery disease, heart attack, heart failure, AFib, and stroke
The risks were 32% higher for coronary artery disease, 47% higher for heart attack, 56% higher for heart failure, 32% higher for AFib, and 28% higher for stroke
The researchers observed a dose-response pattern, with cardiovascular risk generally increasing as nighttime light exposure increased
Still, these were observational findings and cannot establish that light directly caused cardiovascular disease
However, a 2024 review of more than 1 million participants from 14 studies similarly found that greater exposure to light at night was associated with a higher risk of cardiometabolic disease
Circadian rhythm disruption may play a role
The circadian rhythm is the body’s internal 24-hour clock that regulates sleep, hormones, and cardiovascular processes
Light is one of the body’s strongest signals for setting its internal clock. Disruptions from nighttime light exposure could impair circadian signaling and suppress melatonin, potentially interfering with sleep quality
In the new study, shorter sleep was responsible for 24–49% of the associations between nighttime light exposure and several cardiac measurements
Nighttime light can come from televisions, phones, and nightlights, as well as streetlights and illuminated buildings outside the home
Research has shown an association between indoor and outdoor nighttime light and cardiovascular outcomes
“Higher levels of nighttime light exposure have been associated with an increased risk of several cardiovascular conditions, including heart failure, atrial fibrillation, heart attack, and stroke,” Chen said
“Some research has also found an association with a higher risk of cardiovascular death. However, these associations do not necessarily mean that nighttime light directly causes these conditions. More research is needed to better understand the relationship.”
Optimizing your bedroom for quality sleep
More research is needed to determine whether reducing nighttime light directly improves heart health and whether the cardiac changes observed are reversible
Still, there are many benefits to sleeping in a dark bedroom
“Good sleep habits are also important. When possible, keep the bedroom as dark as reasonably possible at night and maintain a consistent sleep schedule,” Chen said
While eliminating all light, even the smallest glow, might not be possible, there are simple strategies to avoid circadian disruption and support quality sleep
- Use blackout curtains or shades if outdoor light enters your bedroom.
- Turn electronics away from the bed and cover unnecessary indicator lights.
- Keep your phone face down or outside the bedroom when practical.
- If you need a nightlight, use the dimmest warm-colored light that provides adequate visibility.
- Maintain relatively consistent sleep and wake times.
- Get bright natural light during the daytime, especially in the morning, to reinforce your circadian rhythm.
Those who live in brightly lit urban environments or experience disrupted sleep may need to take extra measures to optimize their sleep environment
“If avoiding nighttime light is difficult, focus on the cardiovascular risk factors you can control,” Chen said. “This includes eating a heart-healthy diet low in saturated fat and sodium, getting regular physical activity, maintaining a healthy weight, and avoiding tobacco and excessive alcohol.”
