Sleeping with too much light in the room increases the risk of obesity in women, research has shown.
Greater exposure to light at night raised both Body Mass Index (BMI) and waist size in more than 113,000 women taking part in the British study, scientists found.
The Breakthrough Generations 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.
Professor Anthony Swerdlow, from the Institute of Cancer Research in London, said: "Metabolism is affected by cyclical rhythms within the body that relate to sleeping, waking and light exposure.
"The associations we saw in our study between light exposure at night and obesity are very intriguing. We cannot yet tell at this stage what the reason for the associations is, but the results open up an interesting direction for research."
Co-author Dr Emily McFadden said: "Because all the information was collected at the same time, we cannot tell the sequence of events, but the associations we found are consistent with previous research examining light exposure and metabolism, and further investigation is needed."