A recent article by Peter Gary in Psychology Today discussed the rise of mental disorders amongst children correlating with the decline in play.
Pieces that stick out to me is that there is an increase in anxiety or depression in someone's life when they feel a loss of control in their life. I'd tie this to over-scheduling because children have no control over the activity or timing of it.
The other part was that children's goal making skills are now extrinsic instead of intrinsic. The idea that something external, such as a money, is more motivating than intrinsic, I want to do this because I feel good about it.
Do be aware when you read the article, there is a plug for his recent book, Free to Learn. Makes you wonder if the motivation for the book/article is intrinsic or extrinsic?