Sleeping with too much light in the room increases the risk of obesity in women, research has found.
Greater exposure to light at night raised both Body Mass Index (BMI) and waist size in more than 113,000 women, the Breakthrough Generations Study found.
The British study followed the women for 40 years in an attempt to identify root causes of breast cancer. Obesity is a known risk factor for the disease.
"Metabolism is affected by cyclical rhythms within the body that relate to sleeping, waking and light exposure.The results open up an interesting direction for research," Professor Anthony Swerdlow, from The Institute of Cancer Research in London, said.