Researchers from the University of Texas looked at the weight gain of more 18,000 kindergarteners over two years — and made a surprising discovery.

They found that kids’ obesity levels increased only during summer vacations — not during the school year, as some people once thought. During the summer, the number of obese children increased from 8.9 percent to 11.5 percent, while the number of overweight children increased from 23.3 percent to 28.7 percent. Children almost exclusively gained weight during the summer.

Kids often resort to larger amounts of screen time to entertain themselves.

“Our findings raise questions for parents and policymakers about how to help children adopt healthy behaviors during the long summer vacation to stop unhealthy weight gain. Our results also suggest we cannot reverse the obesity epidemic if we focus only on what children are doing and eating while they are in school,” said Paul von Hippel, professor of public affairs at the University of Texas in Austin, in a media release.

The yearly school schedule may pack more physically strenuous activities than the unstructured summer months. Children often participate in sports during the school year, and they keep to regimented schedules for eating and sleeping. Summertime has a looser schedule — and kids often resort to larger amounts of screen time to entertain themselves.

This sedentary behavior combined with unstructured snacking could lead to excessive weight gain. “Parents can take some simple steps to help their children, like sticking to a school-year sleep schedule and reducing screen time,” said Dr. Amanda Staiano of The Obesity Society in Baton Rouge, Louisiana, in a media release.

Related: We Just Keep Getting Fatter

Researchers also suggested restrictions on child-directed food marketing, as well as proactive measures to promote summer school, summer camp, and other outdoor activities. Nutrition education for parents could also be key for healthy children.

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This information comes none too soon. A third of American children are overweight or obese — triple the number of 30 years ago. Childhood obesity can contribute to serious illnesses and health complications, including sleep apnea, liver disease, type 2 diabetes, and cardiovascular disease. It also seriously increases the risk of developing cancer.

Parents can follow the Centers for Disease Control and Prevention recommendations for healthy nutrition by serving reasonable portion sizes, encouraging outside play, reducing fruit juice intake, and limiting the purchase of unhealthy restaurant food.