Does your bedroom stay bright even after you switch off the lights? Street lamps, light filtering through windows or even the glow from electronic devices may be doing more than disturbing sleep, according to a new study, linking greater exposure to light at night with changes in the heart’s structure and functioning.
Researchers studying more than 11,000 people found that those exposed to higher levels of nighttime light had changes in several chambers of the heart, including thickening of the wall of the left ventricle and reduced ability of the heart muscle to flex during a heartbeat.
The findings, published in the European Heart Journal, could help explain earlier observations linking light pollution with cardiovascular disease. But researchers stress that the study was observational and does not prove that nighttime light directly causes heart damage.
The study, led by Professor Lu Qi of Tulane University, New Orleans, included 11,071 participants from the UK Biobank.
Participants wore light sensors on their wrists for seven days to record their exposure to light at night. Three years later, they underwent cardiac magnetic resonance imaging (MRI) to assess the structure and functioning of their hearts.
Researchers compared participants exposed to more than three lux of light at night with those exposed to almost no light.
The difference was notable.
Those with higher nighttime light exposure had a thicker wall in the left ventricle, one of the heart's main pumping chambers. This was associated with less space inside the chamber. Researchers also found signs that the heart muscle was less able to flex during a heartbeat, which can be an early sign of impaired heart function.
Changes were also seen in the right ventricles and the left atria.
“These findings suggest that nighttime light exposure is associated with cardiac remodelling,” Qi said.
Cardiac remodelling refers to changes in the heart's size, shape, structure or function. Such changes can occur when the heart is subjected to prolonged stress or injury.
While the heart can initially adapt to such stress, some forms of remodelling can eventually impair its ability to function efficiently.
Qi said the study was the first of its kind to examine the relationship between nighttime light exposure and these changes in heart structure and function.
“Light pollution has emerged as a new risk factor for cardiovascular disease,” he said, adding that reducing nighttime light exposure could be considered as one potential strategy for preventing heart disease.
However, the researchers' findings need to be interpreted carefully.
The participants were not randomly assigned to sleep in different levels of light. Researchers instead measured their existing exposure and later examined their hearts. This means that other factors linked to lifestyle, health or the environment could partly explain the association.
The study also does not establish what level of nighttime light is harmful to an individual or whether reducing exposure would reverse the changes seen in the heart.
More research will therefore be needed before nighttime light can be considered an established cardiovascular risk factor.
The findings nevertheless raise a practical question: how dark is your bedroom when you sleep?
An accompanying editorial in the journal suggests that doctors, particularly those treating people with heart failure or atrial fibrillation, should ask about their patients' sleeping environment.
Questions could include whether the bedroom is dark, whether the person works night shifts and how much screen exposure they have after sunset.
Some changes suggested by the editorial are simple. Blackout curtains can reduce light entering through windows. Warmer and less intense bedside lighting can reduce exposure before sleep. Small lights from electronic devices can also be covered or switched off where possible.
Cities remain brightly lit through the night, with streetlights, commercial signs, offices, vehicles and other sources contributing to artificial light exposure.
The editorial points to measures such as shielded streetlights, warmer-spectrum bulbs and lighting systems that dim or switch on only when needed. Such measures could potentially reduce unnecessary nighttime illumination while maintaining adequate lighting where it is required.
Artificial light has transformed modern life. Streets can remain illuminated through the night, shops and offices operate long after sunset, and smartphones and other screens keep people exposed to light well into bedtime.
The new study adds the heart to the list of organs researchers are examining in relation to nighttime light exposure.
It does not mean that every person sleeping with some light in the room is at risk of heart disease. Nor does it establish that turning off a bedside lamp will prevent cardiovascular problems.
But it does suggest that the nighttime environment may deserve more attention than it usually receives.
For now, reducing unnecessary light while sleeping is a relatively simple step. Keeping the bedroom as dark as practical, avoiding unnecessarily bright lights before bedtime and reducing screen exposure late at night may help create a better sleep environment, said the researchers.
