While a high-calorie diet and sedentary behavior are the most commonly cited explanations for obesity, some previous research has also linked exposure to artificial light at night to an increased risk of weight gain, researchers note in JAMA Internal Medicine.
For the current study, researchers followed almost 44,000 generally healthy women, ages 35 to 74. Many - about 17,000 - slept with a nightlight in the room, while more than 13,000 left a light on outside the bedroom and about 5,000 slept with a television or light on in the bedroom.
At the start of the study, women were typically overweight but not yet obese, according to their body mass index (BMI), a ratio of weight relative to height. None of them worked shifts that could interrupt sleep cycles.
After almost six years of follow-up, women who slept with a television or light on in the room were 22 percent more likely to be overweight and 33 percent more likely to be obese than women who slept in total darkness without even a nightlight or the glow from an alarm clock.
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.
Still, said lead study author Dr. Yong-Moon Park, "Cutting off lights at bedtime could reduce women's chances of becoming obese."
"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.
"Thus, reducing exposure to artificial light at night while sleeping adds an important tool for preventing weight gain," Park said by email.
Globally, 1.9 billion adults are overweight or obese, according to the World Health Organization. Roughly 4 in 10 adults are overweight, and more than 1 in 10 are obese, a condition that can increase the risk of heart disease, diabetes, kidney problems, joint disorders and certain cancers.
In the current study, women who slept with a television or light on in their room were more likely to have a BMI that put them in the overweight or obese range and to experience at least a 10 percent increase in BMI during the study than women who slept in total darkness.
A BMI between 18.5 and 24.9 is considered healthy, while 25 to 29.9 is overweight, 30 or above is obese and 40 or higher is severely or morbidly obese. (An online BMI calculator is here: http://bit.ly/2Wvl2rq.)
One limitation of the study is that researchers relied on women to report their own height and weight. Researchers also lacked data on any changes in artificial light exposure at night over time that might have impacted the results.
Even so, the results "make a strong case for artificial light exposure at night being a risk factor for weight gain," said James Gangwisch, a researcher at Columbia University in New York City who wasn't involved in the study.
The television and any bedside or overhead lights need to go off, and then people need to look for other light sources to eliminate, he advised.
"Even with the lights off, our bedrooms are often aglow at night from luminous clocks, light-emitting diodes from electronic devices, and outside lighting that seeps through porous curtains and shades," Gangwisch said.