Keeping a lot of light on while you snooze -- such as from a television or bright nightlight -- has been linked with an increased risk of weight gain and obesity.
Specifically, sleeping with a television or light on in the room was positively associated with gaining five kilograms, or 11 pounds, over a five-year period among women in a new study published in the medical journal JAMA Internal Medicine on Monday.
"There was a 17% chance of gaining the five kilograms -- after we adjusted for confounding factors," said Dale Sandler, a senior investigator at the National Institute of Environmental Health Sciences in North Carolina and senior author of the study.
In addition, there was a 22% chance of becoming overweight and a 33% chance of becoming obese, she added.
Obesity means having too much body fat and overweight means weighing too much, according to the US National Library of Medicine. Both overweight and obesity are based on your body mass index but "obese" generally means having a much higher BMI.
"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. "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."
The study involved analyzing data on 43,722 women, aged 35 to 74, in the United States.
The data came from a nationwide cohort study called the Sister Study that enrolled women between 2003 and 2009. The data included information on each woman's sleeping habits, such as if she slept with a small nightlight or a television on, and her body mass index.
Body mass index or BMI, a calculation derived from a person's weight and height, can be used as a screening tool for body fatness and obesity risk. A normal or healthy BMI is typically considered to be between 18.5 and 24.9. A BMI of 30 and higher is considered to be obese.
The women's self-reported sleeping habits were put into four categories: no light, small nightlight in the room, light outside of the room, and light or television in the room.
Women who reported more than one type of artificial light were categorized at the highest level of exposure. Women who slept with a mask on or reported no light while sleeping were classified as experiencing no artificial light exposure.
The researchers took a close look at each woman's sleeping habits and her weight and obesity risk over a five-year period.
Among the women, the researchers found that sleeping with a television or light on in the room was associated with gaining five kilograms or more, a BMI increase of at least 10%, and a higher risk of being overweight or obese, compared with being exposed to no artificial light during sleep.
"There was a dose response, in that the more light in the room the stronger the association," Sandler said.
The study had some limitations, including that only an association was observed in the data -- not a causal relationship. More research is needed to determine whether sleeping with lights on actually could cause weight gain.
"Another limitation is that our data are based on self-reports," Sandler said. The data on artificial light exposure during sleep and weight gain were self-reported, and the women were not asked why they kept lights on while sleeping.
Yet the study findings appear to fall in line with separate research -- including one study, published in the Journal of Clinical Endocrinology and Metabolism in 2016, that linked increased light exposure at night with a 10% increase in body mass index over a 10-year period in older adults.
The new study further highlights "the common recommendation that we make for people to remove TVs and other technology out of the bedroom environment to facilitate healthy sleep," said Dr. Nathaniel Watson, a professor of neurology and director of the Harborview Sleep Clinic at the University of Washington in Seattle, who was not involved in the study.
"As the authors mention, you can't point directly to causality between bedroom light exposure at night for a sleeping individual and weight gain but I think this is definitely a step in that direction," he said. "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."
Professor Malcolm von Schantz, a professor of chronobiology at the University of Surrey in the United Kingdom who was not involved in the study, told the Science Media Centre that the study would have been stronger if the women had been wearing instruments that measured their activity as well as the exact amount of light they were exposed to, rather than depending on self-reports -- "but the findings make perfect biological sense."
"We know that light in the late evening will delay our body clocks. We know from experimental studies in people that light at night affects our metabolism in ways that are consistent with increased risk of metabolic syndrome," he said. "These new findings won't change the advice to maintain good sleep hygiene, and avoid light and electronic distractions in the bedroom, but they add further strength to the case for this advice."