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Dozing off with that phone screen glowing at night isn’t just bad for your sleep — it could be bad for your heart, too.
Exposure to light during sleep, even at levels much lower than a typical smartphone produces, was associated with structural heart changes, according to a new study published in the European Heart Journal on Sept. 9.
The study analyzed the sleep habits of more than 11,000 people over seven days, during which participants wore wrist sensors that measured their light exposure. Participants without heart disease at the beginning of the study then underwent an MRI three years later.
Researchers compared people exposed to no light during sleep with those exposed to 3 lux or more — about the equivalent of moonlight. For comparison, the study authors noted that 1,000 lux generally represents sunlight.
Testing found that those exposed to more light while sleeping showed altered heart function, including thickened muscles and a reduced ability to flex during heartbeat. These signs can be precursors to dysfunction and disease, per the researchers, including heart failure and atrial fibrillation (commonly known as AFib).
“Light pollution has emerged as a new risk factor for cardiovascular disease,” lead researcher and Tulane University professor Lu Qi said in a press statement. “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 policymakers.”
The authors noted several limitations, including limited exposure data because participants wore sensors for only one week, the inability to measure specific types of light such as blue light and the inability to account for differences in individual light sensitivity. Because the study was observational, it cannot prove that nighttime light exposure directly causes heart changes or cardiovascular disease — only that there is an association.
Keeping light at bay before bed
Recent research has supported some association between light during sleep and heart health, including a 2025 JAMA study, a 2022 study published in the Proceedings of the National Academy of Sciences (PNAS) and a 2024 study published in the journal Environmental Pollution.
In an editorial published alongside the latest research, Professor Thomas Münzel from the University Medical Center of Mainz, Germany, offered what he called “simple and inexpensive” advice for limiting light exposure during sleep.
- Using blackout curtains.
- Opting for dimmer, warm-coloured bedside lighting when lights are necessary.
- Covering LED lights on household electronics, including standby or indicator lights.
The National Sleep Foundation likewise suggests limiting exposure to electronics and other bright light sources before bed, including:
- Dimming the lights in your home in the evening as you prepare for bed.
- Turning off screens (including televisions, phones, tablets, computers) at least an hour before bedtime.