Ditto to this. While slacking off during college is not a good idea, working so hard that you burn yourself out is also something to avoid. Believe me, after working on a physics problem for ~20 hours straight it's hard to remember why exactly you ever thought you enjoyed the subject. If you don't take breaks and live life a little it's easy to become disinterested in your field of study, at which point you'll be working hard, learning nothing, and hating every minute of it.

Another thing to keep in mind is that you learn a lot more than academics in college; people also learn a lot socially, which is just as important.