I rarely post links to super-brainy scientific journals, but this one is important enough to wade through. A meta analysis (that's a study of a bunch of studies) shows that risk factors for heart disease later in life are reduced with exercise. We know that. But the study also shows that these risks are increased with sitting around, even when you are young. So it's not a tragedy if your kid is lazy some of the time so long as you get him up and moving regularly in between couch-potato sessions.