Licking late into the night on TV or sleeping with other lights can mix your metabolism and lead to weight gain and even obesity, suggests a preliminary but preliminary US survey.
The National Institutes of Health study published today is not a test but reinforces the evidence suggesting that too much exposure to light at night may pose health risks.
"It's very important that you have this day-night cycle to properly regulate the hormones, the hormones that regulate sleep, the hormones that regulate hunger," said Dale Sandler, a scientist at the National Institute of Environmental Health Sciences. division of the NIH.
"What we contribute is a simple thing that people can do to reduce their chances of becoming obese. It's a very easy prevention effort to just turn off the lights before going to bed," Sandler said.
Daily exposure to light and darkness helps maintain our 24-hour biological clock, which regulates metabolism, sleep-promoting hormones, blood pressure, and other bodily functions.
Mounting research suggests that disrupting this typical sleep-wake cycle can contribute to health problems, increasing the risks of high blood pressure, diabetes, depression, and obesity.
A woman sleeping, holding a tv remote control.
The researchers analyzed health and lifestyle data from nearly 44,000 American women involved in an ongoing study, seeking clues about the causes of breast cancer.
The analysis focused on data on sleep, light exposure and weight gain during the study, but not on breast cancer. The results were published in JAMA Internal Medicine.
The women in the study took medical examinations and completed health and lifestyle questionnaires when they enrolled and then periodically.
Those who reported sleeping at night in a room with a television attached or a light were more likely to gain at least 4.9 pounds in about five years than those who slept in the dark.
They were also about 30% more likely to become obese.
Ms Sandler said she is confident that the additional weight was not for things like evening snacks because the analysis represented other variables that could have led to weight gain such as diet, physical activity and sleep duration.
Sandler said similar results are likely to be found in men.
Animal research and minor studies in humans have linked exposure to prolonged light to weight gain.
Exactly as it is uncertain, but scientists think that disruption in the release of hormones related to sleep and appetite may be involved.