what do you do for fun? do you consider that time wasted?
(honest question. I did used to consider fun stuff to be time wasted, but as I've gotten older and am paying the price for a long-term high stress lifestyle, I'm starting to think differently about it)
Feynman has a fantastic anecdote about getting over what we might today call burnout. He was not interested in work at all in his professorship, when he noticed a wobble in a plate being thrown, and wondered about it. He spent all day working on the physics of this toy problem, and claims discovering play as the key to his recovery.
If you haven't read them yet, "Surely You're Joking, Mr. Feynman", and "What Do You Care What Other People Think" were great reads in my 20s.
The punch line is that the equations he worked out ended up being useful for his Nobel Prize winning work. So it ended up being useful even if that wasn't the goal.