When you think of putting a bit of weight on during the holidays, even with children, you think of the indulgence of Christmas, Thanksgiving and even Halloween. But a new study has found that children are more likely to put on weight during the summer vacation.

A team of researchers specializing in child obesity, found the danger of children gaining weight during summer is much higher – a notion that seems unthinkable since summer seems more about running outside and getting active.

The study took a nationally representative sample of 18,170 kids and analyzed their changes in body mass index (BMI) from kindergarten to second grade. What they found was chances of obesity rose from 8.9 percent to 11.5 percent, and weight gain from 23.3 percent to 28.7 percent. These increases all occurred during the summer months, according to an NPR report.

It is not entirely understood why this happens but it’s thought children are becoming less active and eating more during summers. Other theories are that children’s sleep patterns are disrupted and they actually sleep less which interferes with BMI.

It was thought that less activity during school times was the cause of an increase in weight amongst children but the new study, and other similar ones, all come to the same results. The blame has naturally focused on the healthiness of school meals, physical education and vending machine snacks during school hours but Paul von Hippel, a co-author on the study and an associate professor of public affairs at the LBJ School of Public Affairs at the University of Texas at Austin, says otherwise.

“It really doesn’t appear that schools were ever the problem.”

Von Hippel says popular schemes such as summer camps could provide an opportunity to educate children better and get them more active during the summer months while parents are urged to reduce the amount of time their kids play computer games and watch TV.

Details of the study were published in the journal Obesity.