Sleeping in a room with too much light has been linked to an increased risk of piling on the pounds, a study shows.
A team at the Institute of Cancer Research in London found women had larger waistlines if their bedroom was "light enough to see across" at night.
However, they caution there is not enough evidence to advise people to buy thicker curtains or turn off lights.
The study of 113,000 women was published in the American Journal of Epidemiology.
The women were asked to rate the amount of light in their bedrooms at night as:
Light enough to read
Light enough to see across the room, but not read
Light enough to see your hand in front of you, but not across the room
Too dark to see your hand or you wear a mask
Their answers were compared to several measures of obesity. Body Mass Index, waist-to-hip ratio and waist circumference were all higher in women with lighter rooms.
Prof Anthony Swerdlow, from the Institute of Cancer Research, told the BBC: "In this very large group of people there is an association between reported light exposure at night and overweight and obesity.
"But there is not sufficient evidence to know if making your room darker would make any difference to your weight.
"There might be other explanations for the association, but the findings are intriguing enough to warrant further scientific investigation."
One possible explanation is that the light is disrupting the body clock, which stems from our evolutionary past when we were active when it was light in the day and resting when it was dark at night.
Light alters mood, physical strength and even the way we process food in a 24-hour cycle.
Artificial light is known to disrupt the body clock by delaying the production of the sleep hormone melatonin.
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