The link below will take you to a great article from the January 26, 2010 edition of the New York Times. As with any type of change in the logistics of a school day, there are always issues that need to be addressed in order for the change to be effective.
During my 20 plus years of teaching elementary/middle school, this thought never came to mind. However, I did implement a 15 minute "cool down" period when my students returned from the lunch/playground time (lunch first then playground). It worked wonders for me with respect to them being more calm and allowing for an easier transition into focusing on academics for the afternoon.
Parents may find this useful to implement when their child/children get home from school...a little play time before beginning homework or a class project.
Enjoy the article, please share your thoughts!