Nighttime light may be quietly troubling your heart

A new study links even a dim bedroom glow to subtle changes in heart structure, and doctors say darkness deserves more credit than it gets

Most sleep advice comes down to a familiar formula: get at least seven hours and keep the bedroom cool and dark. New research suggests that last part may matter for more than a good night’s rest.

A study published in the European Heart Journal connects exposure to light at night, from hallway fixtures and night-lights to the streetlamp glowing past the curtains, with early changes in the heart that could raise the odds of cardiovascular disease. City dwellers are not destined for heart trouble, doctors say, but the findings strengthen the argument for shutting out as much light as possible after bedtime.

What researchers measured

The team drew on data from 11,071 participants in the UK Biobank, a large biomedical database. Each person wore a wrist sensor for a week that tracked how much light reached them overnight.

Three years later, participants underwent cardiac MRI scans so researchers could examine the structure and performance of their hearts. The group was then split in two: people exposed to more than three lux of light at night, roughly the brightness of bright moonlight or the sliver that slips under a door, and people who slept in near-total darkness.

How light showed up in heart scans

Those in the brighter group showed thickening in the wall of the left ventricle, the heart’s main pumping chamber, which left less room inside it. Their scans also revealed heart muscle that flexed less with each beat, a possible early signal that the organ is not working as efficiently as it should.

The authors concluded that light exposure at night may raise cardiovascular risk, and they pointed to dark nights and sufficient sleep as worthwhile priorities for heart health.

The study does not stand alone. Research presented at the American Heart Association’s Scientific Sessions 2025 found that people exposed to more artificial light at night had elevated stress activity in the brain, inflammation in their blood vessels and a modestly higher risk of major cardiac events, including heart disease.

Why the body treats light as an alarm

Doctors point to several possible explanations. Dr. Christopher Winter, a neurologist and sleep medicine specialist who wrote The Sleep Solution: Why Your Sleep Is Broken and How to Fix It, explained that the brain reads light as a cue to wake up. Light at night, he said, pits a brain ready for rest against incoming signals suggesting the timing is wrong.

That tension can throw off the circadian rhythm, the internal clock that helps govern sleep, blood pressure, metabolism and other functions tied to heart health, according to Dr. Yu-Ming Ni, a cardiologist and lipidologist at MemorialCare Heart and Vascular Institute at Orange Coast Medical Center in Fountain Valley, California. Over the long haul, Ni said, disrupted sleep and a scrambled body clock may feed into risk factors such as high blood pressure, obesity and diabetes.

Sleep also gives the cardiovascular system a chance to recover. During healthy rest, heart rate and blood pressure typically dip, allowing the heart and blood vessels to bounce back from the strain of the day, Ni noted. When sleep is regularly interrupted or cut short, that recovery may suffer, adding stress to the system over time.

Still, experts caution that the new study identified an association between light at night and heart disease risk factors. It did not show that one causes the other.

Simple ways to sleep in the dark

Winter said more research is needed to pin down what drives the connection, but he recommends sleeping in darkness regardless. Experts suggest these habits for reducing light exposure at night:

• Consider wearing sleep masks
• Use blackout blinds and window coverings
• Dim the lights in your home in the evening
• Limit bright screens before bed
• Turn off unnecessary lights at night

The aim is not flawless darkness at any cost, Ni said, but cutting out needless light and building a sleep space that feels comfortable and restful. For a habit that costs little more than a pair of blackout curtains, the potential payoff for the heart may be worth the effort.

Source: Women’s Health