Hi, during activity or exercising physically children tire or experience fatigue, hunger or dehydration which impacts on their ability to focus and concentrate. Once rested their motivation and enjoyment of activity enables them to explore using all of their skills, physically and emotionally. The amount of rest and quiet times children need varies depending on age -
Google's cached page of NHS choices
page 239 & 340 of the
Heinneman Lv 2 handbook will help more
Best wishes