They were also about 30 percent more likely to become obese.
Daily exposure to light and darkness helps maintain our 24-hour body clock, which regulates metabolism, sleep-promoting hormones, blood pressure, and other bodily functions.
Poor sleep as a standalone factor has been proven to be associated with obesity and weight gain, but Dale Sandler, Ph.D. and a co-author of the research, said, "it does not explain the associations between artificial light exposure during sleep and weight gain".
According to scientists, women who are exposed to artificial light in the evenings are more likely to gain weight.
They were also unable to disentangle the relationship between ALAN and factors such as an unhealthy diet, sedentary lifestyle, stress and other sleep characteristics - nor did they ask women why they slept with a light on.
The participants, aged 35-74 years, had no history of cancer or cardiovascular disease and were not shift workers, daytime sleepers, or pregnant when the study began.
The study researchers found that women who reported exposure to light at night while sleeping were more likely to gain weight and become obese over almost six years, compared with women who were not exposed to light at night.
Light coming in from outside the room was associated with more modest weight gain, researchers found. Shorter sleep could prompt you to exercise less and eat more, he noted.
"Exposure to artificial light at night may alter hormones and other biological processes in ways that raise the risk of health conditions like obesity".
"Our findings. suggest that lowering exposure to [artificial night at light] while sleeping may be a useful intervention for obesity prevention", researchers said. A study followed nearly 44,000 women for five years and those who left lights on at night gained up to 11lbs (5kg).
"There was a dose response, in that the more light in the room the stronger the association", Sandler said. Health officials recommend taking TVs and other tech devices out of your bedroom in order to support a healthy sleeping environment. That being said, a direct link between artificial light in a bedroom and overall weight gain is still a bit startling to see. "It indicates that we need to respect our sleep and respecting our sleep means making a sleep environment devoid of any type of light ideally".
"We know that light in the late evening will delay our body clocks".
The National Institutes of Health study published Monday isn't proof, but it bolsters evidence suggesting that too much exposure to light at night could pose health risks.
Dr. Phyllis Zee, an expert in sleep and circadian rhythm disorders at Chicago's Northwestern University, said the study is important because it highlights a behavior that can be easily changed to reduce the risk of gaining weight.