In addition, there was a 22% chance of becoming overweight and a 33% chance of becoming obese, she added.
“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 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.
“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.”
“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.”