Sleeping with a light on could be bad for your heart. Researchers in the USA say even a light that is not very bright could be harmful. The researchers did tests on 11,000 people. The people wore sensors that measured the amount of light in their bedrooms. People who had no heart disease had an MRI scan three years later. The scans showed a link between sleeping with a light on and changes in the structure of the heart. One change was thicker heart muscles. Another change was the heart was less able to pump blood, stretch, and relax during heartbeats.

The study is published in the European Heart Journal. The lead researcher said light pollution was a new risk factor for heart disease. He suggested that reducing light while sleeping could be a strategy for preventing heart disease. The researchers advised people to turn off lights, televisions, and smartphones in their bedroom. They said people needed to make the room as dark as possible. They added: "The advice is simple and inexpensive: blackout curtains, warm-coloured bedside lighting, and covering the small standby LEDs on household electronics."