Maybe it’s not so much what you eat but how dark it is when you sleep that is affecting your weight.

A study by scientists at the National Institutes of Health shows a link between artificial light at night while sleeping and weight gain. The results suggest that cutting all lights during sleep could lower women’s chances of obesity.

The researchers used data from a questionnaire sent to around 40,000 women in the Sister Study, a cohort study that examines risk factors for breast cancers and other diseases. The participants were ages 35 to 74, had no history of cancer or cardiovascular disease and had normal sleep habits.

The researchers asked whether they slept with no light, a small nightlight, light outside of the room, or a light or a turned-on television. They also gathered data including weight, height, BMI and waist-to-hip circumference.

Then, they did a follow-up questionnaire five years later. They found the results varied according to the level of exposure to artificial light at night. For example, using a nightlight had no effect, but those who slept with an artificial light or with the television on were 17% more likely to have gained 11 pounds or more.

Humans, the study noted, are genetically adapted to a natural environment consisting of sunlight during the day and darkness at night. Exposure to artificial light at night may alter hormones and other biological processes in ways that raise the risk of health conditions like obesity.

To get a good night’s sleep, use a nightlight if you have to, switch devices to night-time mode, read a book or try using lavender oil. Sweet dreams!