Researchers say sleeping with a light on could harm your heart. Even lights that are not so bright could be harmful. The researchers put sensors on 11,000 people to measure the light in their bedrooms. People with 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 heart. The heart muscles were thicker. Also, the heart was less able to pump blood.

The lead researcher said light was a new risk for heart disease. He suggested that reducing light while sleeping could help to prevent heart disease. The researchers advised people to turn off lights in their bedroom. They said the room should be as dark as possible. They added that their advice was simple and cheap: use blackout curtains and warm-coloured lights. They also suggested covering the small standby LEDs on household devices.