But some researchers now say 20 minutes a day can be just as effective when it comes to lowering the risk for type 2 diabetes.
Researchers at the Medical College of Georgia put more than 200 overweight kids in a study. Some exercised 40 minutes a day, some 20 minutes a day, and others less.
Both the 20 and 40-minute groups improved their fitness and lowered their insulin resistance. That is a key factor in type 2 diabetes.
Both groups also lost fat around the internal organs, however the 40-minute group lost the most.
Still, researchers say showing how 20 minutes makes a difference, gives schools more incentive to fit fitness into the schedule.