For me at least, there was little correlation between level and pure number of hours worked. In general, I probably worked the most hours in the first year, mostly because I was just out of college and didn't know any better. These days, I would say I probably average around ~50 hours a week.

But there is definitely more "stress" as you go up levels. You have more responsibility, the problems get harder and more ambiguous, more people issues to deal with, etc. Part of me misses the "L3/L4 me", when all I really had to do was come in, work on relatively scoped technical problems, and not worry about a lot else. But I think I'd also probably feel bored after a while if I somehow managed to went back to that.