Too much light at night may quietly affect your heart

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By Sola Ogundipe

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If your bedroom isn’t completely dark at night, a new study suggests it may be worth taking a closer look at how much light reaches you while you sleep.

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Research involving more than 11,000 people found that higher exposure to light at night was associated with subtle changes in the heart, including thicker and stiffer heart muscle.

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The study doesn’t prove that nighttime light causes these changes. It does, however, add to growing evidence that exposure to light after dark may have implications for cardiovascular health.

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Tonight, switch off unnecessary lights, draw the curtains, cover bright LEDs and put the phone away before bed. Keep the room as dark as comfortably possible.

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The findings, published September 9 in the European Heart Journal, raise fresh questions about something most people rarely think about: how much artificial light surrounds us after sunset.

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The research was led by Professor Lu Qi of Tulane University, New Orleans, USA. He noted that previous observational studies had linked nighttime light exposure with a higher risk of cardiovascular disease, but much less was known about what might actually be happening inside the heart.

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“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,” Qi said.

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The study involved 11,071 participants from the UK Biobank. Each wore a light sensor on the wrist for seven days, giving researchers a measure of how much light they encountered during the night.

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About three years later, the participants underwent cardiac magnetic resonance imaging, or MRI. The scans gave researchers a detailed look at the structure of the heart and how well it was functioning.

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The researchers compared people exposed to nighttime light levels above three lux with those exposed to almost no light at night. One of the clearest differences was found in the left ventricle, the heart’s main pumping chamber.

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Those exposed to higher levels of nighttime light had thicker walls in the left ventricle. As the wall thickened, the space inside the chamber became smaller. The scans also showed that the heart muscle was less able to flex normally with each heartbeat.

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Reduced flexibility can be an early sign of altered cardiac function. Changes were also found in the right ventricles and left atria, suggesting that the association was not confined to a single chamber of the heart.

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The heart has to contract and relax with every beat. When the muscle becomes thicker and less flexible, it can become harder for the heart to relax properly and fill with blood.

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Qi described the structural and functional changes as cardiac remodelling, a process in which the heart changes in response to prolonged stress or injury.

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“This is the first study of its kind,” he said, adding that cardiac remodelling may initially be the body’s way of adapting to stress but could ultimately weaken the heart and contribute to heart failure. The researchers said the findings add to evidence suggesting that light pollution could be an emerging cardiovascular risk factor.

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There is, however, an important caveat. The study found an association between nighttime light exposure and cardiac changes. It did not establish that exposure to light directly caused those changes. Other factors could have played a role, so the findings will need to be confirmed by further research.

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Could a darker bedroom help? An accompanying editorial by Professor Thomas Münzel of University Medical Center Mainz, Germany, and colleagues argues that doctors may need to pay greater attention to how much light their patients encounter at night, particularly those already living with heart failure or atrial fibrillation.

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They suggested that doctors ask about the sleep environment: how dark the bedroom is, whether the patient works night shifts and how much screen exposure occurs after sunset. The proposed changes are simple and inexpensive. They include blackout curtains, warmer and dimmer bedside lighting, and covering bright standby lights on household electronics.

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But the issue goes beyond the bedroom. Modern cities remain illuminated long after sunset, with streetlights, buildings, shops, advertising signs and vehicles contributing to the glow. The editorial authors argue that cities can reduce unnecessary exposure through measures such as shielded light fixtures, warmer-spectrum bulbs and dimming or motion-activated streetlights.

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For individuals, though, the first step may be much simpler. Switch off lights you don’t need. Draw the curtains. Cover bright LEDs. Dim your screens and put the phone away before bed. Then keep the bedroom as dark as comfortably possible.

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So tonight, take a look around your bedroom. Turn off the lights you don’t need, draw the curtains, cover bright LEDs and put your phone away before bed. Keeping your sleeping space darker is a simple step you can take while researchers continue to study the possible heart effects of nighttime light.

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Whether such measures can actually prevent heart disease remains to be established. Still, as nights become brighter, researchers are asking a question worth taking seriously: what happens to the human heart when the world around us never quite switches off?

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