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. The few studies that have been conducted in the general population have typically collected data at a single point in time, so researchers haven't been able to determine whether light at night is tied to weight gain over time.
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.
The study followed 43,722 women aged between 35 and 74 years old, over a period of at least five years.
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.
There was also a 22-percent chance of becoming overweight and a 33-percent chance of becoming obese or having too much body fat.
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.
The authors said, "These results suggest that exposure to artificial light at night while sleeping may be a risk factor for weight gain and development of overweight or obesity".
"Turning off the light while sleeping may be a useful tool for reducing a possibility of weight gain and becoming overweight or obese", said lead author Dr. Yong-Moon Mark Park.
One limitation of the study is that researchers relied on women to report their own height and weight.
Park said that exposure to artificial light at night may suppress the sleep hormone melatonin and disrupt the natural sleep-wake cycle.
Researchers also noted other factors, like exposure to artificial light at night can be reflective of unhealthy behaviour, such as eating badly, sedentary lifestyle or stress, and socioeconomic disadvantage. We know that light in the late evening will delay our body clocks.
Commenting on the paper, Malcolm von Schantz, a professor of Chronobiology at the University of Surrey in Britain said: "What is novel with this paper is that it is a longitudinal study comparing the weight of the same individuals at baseline and more than five years later". "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.