A recent study made by researchers at the University of Michigan’s Center for Human Growth and Development revealed that children aged 9 to 12 who sleep less then nine hours a night can have real problems with gaining weight.

Dr. Julie Lumeng, lead author of the report said: “Many children aren’t getting enough sleep, and that lack of sleep may not only be making them moody or preventing them from being alert and ready to learn at school, it may also be leading to a higher risk of being overweight,” Reuters reports.

She also said that children who use to sleep more than 10 hours a night are more willing to play and consume their energy, rather than staying in bed or watching TV like their tired fellows do.

“I’m a parent, too, so I understand that these kids are so irritable that their parents are more apt to say, ‘Here, go ahead and have a cookie. Just chill out,’ “she added.

There are also hormonal explanations for children gaining weight.

Endocrinologist Eve Van Cauter of the University of Chicago said that adults who do not sleep enough produce more ghrelin, a hormone that gives the sensation of hunger and less leptin, a hormone that signals fullness, the Associated Press reports.

The study focused on 785 children aged 9 to 12 from 10 different states and it confirmed that eighteen percent were overweight by the time they reached the sixth grade. Children who do not sleep nine or more hours a night when they are in the third grade are more likely to gain weight by the time they are in the sixth grade.

The conclusion of the study is very simple:

“Our findings also provide additional support for policies that propose later school start times. The very early school start times for U.S. adolescents have raised concerns in the pediatric community because of their apparent adverse impact on sleep duration and, consequently, children’s general academic and behavioral functioning,” Reuters reports.

According to National Sleep Foundation, children in first grades should sleep from 10 to 12 hours a night.

Dr. Lumeng’s report was published in the November “Pediatrics,” the journal of the American Academy of Pediatrics.