But don't go out and spend money on changes just yet. There is not enough evidence to make you buy thicker curtains or turn off lights.
Funded by Breakthrough Breast Cancer, 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.
That's amazing. All? Sounds conclusive to me.
But the evidence from such a large group of people links light exposure at night with overweight and obesity. Artificial light is known to disrupt the body clock by delaying the production of the sleep hormone melatonin.
One explanation could be the light disrupts the body clock. In our evolutionary past, we were active when it was light and resting when it was dark.
Here's another way of modern suburban living affecting people in a bad way. In society's artificial environment, the combination of street lights, traffic whizzing by at all hours of the night, and electrical equipment could be affecting the way our body processes food.