It found those who said they sleep with a TV or a light on in the room gained 11 pounds or more, and had a body mass increase of about 10 percent over a 5-year period. Previous studies in animals have suggested that exposure to light at night may disrupt sleep and circadian rhythms, alter eating behaviors, and promote weight gain, the authors said. They were also about 30% more likely to become obese.
The study wasn't a controlled experiment and so it can't prove whether or how exposure to artificial light at night might directly cause obesity.
Obesity means having too much body fat and overweight means weighing too much, according to the US National Library of Medicine. "If these study findings are true and if they can be replicated then it's a very easy public health message to turn off the lights when you're sleeping".
"We are in the middle of an obesity epidemic in the United States and the things that we usually think about for obesity prevention are hard for people to do - eat a better diet, get more exercise - and we don't seem to be making a dent", Sandler said. "This study highlights the importance of artificial light at night and gives women who sleep with lights or the television on a way to improve their health".
The researchers considered several other compounding factors, like sleep deprivation, which could have played a role in the association between artificial light exposure at night and weight gain.
Eliminating lights and screens from bedrooms could be an important step in fighting the obesity crisis, the researchers believe.
"In our study exposure to artificial light at night while sleeping was associated with weight gain both in women with insufficient sleep - less than seven hours - and women with sufficient sleep - seven to nine hours", said Park, a researcher with the National Institute of Environmental Health Sciences at the National Institutes of Health in Research Park Triangle, North Carolina.
Women who reported more than one type of artificial light were categorized at the highest level of exposure. "We know that light late at night delays our internal clocks", he wrote in a comment.
However, commenting on the study, Professor Malcolm von Schantz, from the University of Surrey, said, "The findings make flawless biological sense".
This sleep study comes on the heels of another study that found irregular sleep patterns, including not going to bed and waking up at the same time each day or getting different amounts of sleep each night, can put people at a higher risk for obesity, heart disease, hypertension, high blood sugar and other metabolic disorders.