The study, which focused on women between the ages of 35 and 74, shows that sleeping with the TV on - or any other source of bright light in the bedroom - and gaining weight tend to go hand-in-hand.
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, provocative but preliminary US research suggests.
The group provided information on their weight, Body Mass Index (BMI), and any exposure to artificial light during the night.
The National Institutes of Health study published Monday isn't proof, but it bolsters evidence suggesting that too much exposure to light at night could pose health risks.
Although the new findings aren't conclusive, reducing your exposure to light and night may not be a bad idea. "It seems reasonable to advise people not to sleep with lights on", Park and Sandler said. "Further prospective and interventional studies could help elucidate this association and clarify whether lowering exposure to ALAN while sleeping can promote obesity prevention". 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 researchers analyzed health and lifestyle data on almost 44,000 USA 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.
They were also about 30 per cent more likely to become obese.
The findings held even after the researchers took into account factors such as where participants lived (in an urban, suburban or rural area), their household income, their level of caffeine and alcohol consumption, and any experiences of depression or high stress.
"Evolutionarily we are supposed to be sleeping at night, in a dark place", said lead author Dale Sandler, a scientist with the National Institute of Environmental Health Sciences, a division of the National Institutes of Health.
Other possibilities were that light acts as a "chronic stressor" disrupting the release of stress hormones such as glucocorticoids that play a part in regulating food intake, or that there may be another mechanism at work that affects metabolism directly.