But new research points to another potential cause: how much light you're exposed to at night.
In the new study, the researchers analyzed information from almost 44,000 women ages 35 to 74 from all 50 US states. Participants fell between the ages of 35-74, had no history of cancer or cardiovascular disease, and were not shift workers, daytime sleepers, or pregnant at the time of the study.
They were also unable to disentangle the relationship between ALAN and factors such as an unhealthy diet, sedentary lifestyle, stress and other sleep characteristics - nor did they ask women why they slept with a light on.
The study by the American Medical Association found a link between sleeping with the television or light on, and weight gain.
The study researchers found that women who reported exposure to light at night while sleeping were more likely to gain weight and become obese over almost six years, compared with women who were not exposed to light at night.
The study found that women were 17 percent more likely to have gained 11 pounds or more over a five-year period if they slept with a television or other light on in the room than those who slept in dark rooms or with a small nightlight.
"Humans are genetically adapted to a natural environment consisting of sunlight during the day and darkness at night", Chandra Jackson, Ph.D., and the study's co-author said.
"These new findings won't change the advice to maintain good sleep hygiene, and avoid light and electronic distractions in the bedroom, but they add further strength to the case for this advice".
The authors acknowledge that other confounding factors could explain the associations between artificial light at night and weight gain.
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.
"The two most common strategies for obesity prevention are promoting healthy diets and increased exercise levels", researchers said.
"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.
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