NEW ORLEANS, United States — Sleeping with even a small amount of light in the bedroom may affect the structure and function of the heart, potentially increasing the risk of future cardiovascular disease, according to a new study.
Researchers found that people exposed to more than 3 lux of light while asleep had thicker heart muscle walls and changes that could reduce the efficiency of the heart’s pumping action.
The findings, published in the European Heart Journal, add to growing evidence that nighttime light exposure may be an overlooked cardiovascular risk factor.
“Light pollution has emerged as a new risk factor for cardiovascular disease,” said senior study author Dr Lu Qi, the HCA Regents Distinguished Chair and professor and director of the Tulane University Obesity Research Center in New Orleans.
“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,” Qi said in a statement.
How nighttime light may affect the heart
For quality sleep, experts recommend keeping bedrooms very dark, ideally at 1 lux or less.
Lux is a unit used to measure illumination. About 3 lux is comparable to moonlight or light entering a room beneath a closed bedroom door, according to experts.
In the study, researchers found that exposure to more than 3 lux while sleeping was associated with thickening of the heart muscle walls.
The changes appeared to reduce the amount of space inside the left ventricle, the heart chamber responsible for pumping oxygen-rich blood around the body.
Researchers also found a reduction in the heart muscle’s ability to flex during a heartbeat, which can be an early indicator of impaired heart function.
Additional changes were identified in two other chambers of the heart.
The researchers found that the extent of the changes increased as nighttime light exposure became brighter.
Potential sources of exposure include the glow from a face-up smartphone, a digital alarm clock or outdoor light pollution entering through windows.
Thicker heart muscle may lead to heart problems
The reshaping of the heart muscle is known as cardiac remodeling, according to Dr Andrew Freeman, director of cardiovascular prevention and wellness at National Jewish Health in Denver.
“We don’t want the heart muscle to get thick. We don’t want the heart muscle to get stiff. We don’t want the heart to get too big,” Freeman said.
He added that these changes appeared to occur in association with excess light exposure and potentially poor-quality sleep.
Freeman was not involved in the study.
The researchers analysed more than 11,000 people who did not have cardiovascular disease and were participants in the UK Biobank, a large biomedical database containing genetic and health information from about 500,000 people in the United Kingdom.
Participants wore wrist sensors for seven days to measure their exposure to light during nighttime sleep.
Three years later, they underwent cardiovascular magnetic resonance imaging to assess the structure and function of their hearts.
Shorter sleep explains part of the effect
The study also examined whether shorter sleep duration could explain some of the association between nighttime light exposure and heart changes.
Qi said between 24pc and 49pc of the impact observed on the heart was attributable to shorter sleep lasting less than five or six hours.
He described reduced sleep as a direct consequence of nighttime light exposure.
Other factors could account for the remaining risk, including possible effects on stress, blood pressure and heart rate.
“Night light exposure may affect other cardiovascular disease risk factors such as stress, biological changes,” Qi said in an email.
Study has important limitations
Researchers cautioned that the findings do not establish that nighttime light directly causes changes in the heart.
Karl Lawrence, a lecturer at the Institute of Pharmaceutical Science at King’s College London, said the study had several limitations.
One concern was that participants’ light exposure was measured for only one week, which Lawrence said was a relatively short period for assessing overall nighttime exposure.
He also questioned whether a light sensor worn on the wrist accurately represented the amount of light reaching the eyes.
The sensor, he said, could result in either an overestimation or underestimation of the actual exposure received by the eye.
“To correlate this to heart scans three years later feels like it is somewhat of a stretch,” Lawrence said in an email.
He added that further studies would be required to demonstrate causality.
The distinction is important: the study found an association between nighttime light exposure and changes in heart structure and function, but it does not prove that light exposure itself caused those changes.
How to reduce light exposure while sleeping
Experts say reducing nighttime light exposure is relatively simple.
Heavy curtains, blackout curtains or shades can help prevent outdoor light from entering the bedroom.
People should also open curtains or blinds in the morning to expose themselves to daylight, which helps regulate the body’s circadian rhythm.
Hallway lights should be switched off at night where possible.
If light enters beneath doors or around window edges, light-blocking foam or strips can be used to reduce the amount entering the room.
A sleep mask may also help if low levels of light remain.
Choose warmer, dimmer bedroom lighting
For people who need a bedside reading light, experts recommend bulbs with warm white, deep amber or red tones and low or near-zero effective colour temperatures.
If a night-light is necessary, it should be kept dim and positioned close to the floor.
Bulbs with reddish or brownish tones may be preferable for nighttime use.
The study adds to evidence that the bedroom environment may play an important role in sleep and cardiovascular health.
While more research is needed to determine whether nighttime light directly causes harmful changes to the heart, reducing unnecessary light during sleep is a relatively straightforward step that may support better sleep quality.
