To get a better idea of how artificial light exposure at night affects women's weight, researchers logged participants' weight, height, waist and hip circumference, and body mass index measurements at the beginning of the study, and compared it to the same data gathered five years later.
Published by the Journal of the American Medical Association, the study was based on a survey of almost 44,000 American women, who were followed for five years. The women, who were enrolled in the Sister Study group, had no history of cancer or cardiovascular disease and weren't shift workers, daytime sleepers or pregnant at the study's start.
Women exposed to artificial light at night were more likely to gain weight and become obese or overweight over the next five years, according to a study of nearly 44,000 people.
In other words, exposure to light at night might represent a "constellation" of factors, including those related to unhealthy behaviors, "all of which could contribute to weight gain and obesity", the authors said.
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. One of the most important findings was that women who slept with a light or with the television on in the room were 17% more likely to have gained 5 kilos or more during the investigation period.
They suggested that light could be suppressing melatonin production, which alters the circadian rhythm and eating patterns.
"Humans are genetically adapted to a natural environment consisting of sunlight during the day and darkness at night", Chandra Jackson, Ph.D., and the study's co-author said.
A few potential culprits to weight gain were at play.
Controlling calories and getting as much physical activity as possible should still be the top of everyone's list if you're looking to manage weight, but this researcher shows there are other things to consider as well. Also, the study did not include men. "It seems reasonable to advise people not to sleep with lights on", Park and Sandler said.
For middle-aged and older women, sleeping with the television or lights on has been linked with higher odds of becoming obese, in a recent US study.