Cross-posted from my studio blog: (http://guesspianostudio.weebly.com/1/post/2013/06/practicing.html)
This was written by a violinist, but is very applicable to pianists. Deliberate, intentional, reasonable practice is so, so important to musical improvement! When I was in college, I practiced 8-10 hours a day during some periods, and I paid for it in unpleasant physical symptoms. Practicing too much is an issue, especially for more advanced students; as I tell my students, practice smart!
This article is well worth the read.