"Humans are genetically adapted to a natural environment consisting of sunlight during the day and darkness at night", Chandra Jackson, Ph.D. and co-author of the study explained, "Exposure to artificial light at night may alter hormones and other biological processes in ways that raise the risk of health conditions like obesity". Sleeping with a light on outside the room resulted in a small weight gain, according to the study.
The researchers analysed health and lifestyle data on almost 44,000 U.S. women enrolled in an ongoing study seeking clues to causes of breast cancer.
Women who slept with a TV or light on in the room were 17% more likely to gain at least 11 pounds (5 kilograms) during the study, compared with the no-night-light group.
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.
"Although poor sleep by itself was associated with obesity and weight gain, it did not explain the associations between exposure to artificial light while sleeping and weight", said corresponding author Dale Sandler, Ph.D., chief of the Epidemiology Branch at the National Institute of Environmental Health Sciences (NIEHS), part of NIH.
They were also about 30 per cent more likely to become obese.
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One limitation of the study is that researchers relied on women to report their own height and weight.
Dozing off to late-night TV or sleeping with other lights on may mix up your metabolism and lead to weight gain and even obesity, a provocative but preliminary United States research suggests. "We know that light late at night delays our internal clocks", he wrote in a comment.
The study wasn't a controlled experiment and so it can't prove whether or how exposure to artificial light at night might directly cause obesity. The women also answered questions about their level of exposure to light at night while sleeping, such as light from other rooms, light from outside, light from a TV, or light in the bedroom.
Even so, the results "make a strong case for artificial light exposure at night being a risk factor for weight gain", said James Gangwisch, a researcher at Columbia University in New York City who wasn't involved in the study.
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 flawless biological sense".