Girls and young women who use the Internet a lot, get too little sleep or drink alcohol regularly, are likelier than their peers to put on excess weight, a new study suggests.

The researchers, who tracked more than 5,000 girls between 14 and 21 years old for one year, found that the more spare time girls spent on the Internet, the more their body mass index (BMI) increased.

The same pattern was seen when the team looked at alcohol consumption and sleep. In fact, lack of sleep was linked to greater gains in BMI - a measure of weight in relation to height.

The findings, in the Journal Of Pediatrics, add to evidence fingering each of these three habits in promoting weight gain.

The effect of each may be small but. over time, the kilos can add up, said the researchers led by

Dr Catherine Berkey of Brigham & Women's Hospital and Harvard Medical School in Boston.

The study involved 5,036 girls and young women. They were surveyed on the number of recreational hours per week they spent on the Internet (from one to five hours to 16-plus hours), as well as how long they typically slept each night (anywhere from five hours or less, to nine hours or more) and how much alcohol they usually drank (ranging from none to two or more drinks per week).

The researchers found that, generally, as Internet use climbed, so did the BMI, particularly among girls younger than 18 years old.

When it came to sleep, those who clocked five hours or less tended to gain more weight than those who got the standard eight hours. Girls and women who had two or more alcoholic drinks per week put on more kilos than those who drank the least.

The effects over one year were modest, the team said. For example, a 19-year-old of average weight and height would gain about 1.8kg if she were in the high-risk groups for Internet use, sleep and alcohol consumption.

However, the researchers added, over time that could translate into significant weight change.

Spending more hours on the Internet was likely to contribute to weight gain by taking time away from physical activity, Dr Berkey's team said.

Lack of sleep may make people too tired to be active during the day; lack of sleep also affects hormones and metabolism in a way that might boost weight gain.

Meanwhile, alcohol has a large amount of calories and research suggests that people usually don't adjust for liquid calories by eating or drinking less throughout the rest of the day.