While the study doesn't prove that sleeping with a light on causes weight gain, it suggests the two may be linked, the researchers said. "Further prospective and interventional studies could help elucidate this association and clarify whether lowering exposure to ALAN while sleeping can promote obesity prevention".
For the study, Park's team relied on self-reported data from almost 44 000 women, aged 35 to 74.
Keeping a light on might also result in poorer sleep.
Ashton suggests that women create a prime sleeping environment for themselves by using tools like eye masks and blackout shades or drapes.
Park said that exposure to artificial light at night may suppress the sleep hormone melatonin and disrupt the natural sleep-wake cycle. "It is a medical necessity on par with our food and our fitness".
Based on a survey of 43,722 women aged 35-74 with no history of cancer or cardiovascular disease, who weren't shift workers, daytime sleepers or pregnant at the start of the study, the scientists asked about how much light inside and outside the room participants slept with.
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. "This study highlights the importance of artificial light at night and gives women who sleep with lights or the television on a way to improve their health". Sleeping with a light on outside the room was associated with only slight increases in weight. However, their findings did not change when analyses controlled for characteristics that may be associated with exposure to light at night.
The research showed that women who slept with a television or other light on in the same room at night were 17 percent more likely to have gained 11 pounds or more over the five-year follow-up period than those who slept with a small night light.
Ms Sandler said she is confident that the added weight wasn't from things like snacking at night, because the analysis accounted for other variables that could have led to weight gain such as diet, physical activity and sleep duration. Also, the study did not include men.
Getting enough quality sleep every night is a tip at the top of any healthy living checklist, and lack of sleep has indeed been associated with a variety of conditions including obesity.
About the National Institute of Environmental Health Sciences (NIEHS): NIEHS supports research to understand the effects of the environment on human health and is part of the National Institutes of Health. "It seems reasonable to advise people not to sleep with lights on", Park and Sandler said. NIH is the primary federal agency conducting and supporting basic, clinical, and translational medical research, and is investigating the causes, treatments, and cures for both common and rare diseases.