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The study, conducted at the National Institutes of Health, is the first to suggest an association between exposure to artificial light at night while sleeping and weight gain. The researchers believe total darkness at bedtime could reduce the chances of becoming obese.

The results varied with the level of artificial light exposure. Using a small nightlight was not associated with weight gain, whereas those who slept with a light or television on were 17% more likely to have gained five kilograms or more over five years.

The team looked at data from over 40,000 women. The participants were aged 35-74, had no history of cancer or heart disease, and were not shift workers or pregnant when the study began. Women were asked whether they slept with no light, a small nightlight, light outside of the room, or a light or television on in the room. Weight, height, waist and hip circumference and body mass index (BMI) measurements were recorded at the start of the study, and five years later at the end.

Dr Chandra Jackson, head of the NIEHS Social and Environmental Determinants of Health Equity Group, and one of the study's co-authors said: "Humans 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."

Of course, other factors may explain the association between artificial light and weight gain. But the researchers say their findings did not change when they controlled the data for characterisations that may be associated with artificial light exposure at night or weight gain, such as age, children in the home, race, calories consumed and physical activity.

Dr Michelle Miller, associate professor of biochemical medicine at the University of Warwick, said the research was promising but pointed out there were a number of limitations with the methods used.

"The study was only conducted in women; measures of light exposure were self-reported and non-quantifiable and sleep duration was also self-reported. The findings are however consistent with current advice that sleeping environments should be as dark as possible."

The research was published in JAMA Internal Medicine.