I've been working 10 hour days for the last 4 years, and I consistently come in under-budget, on time, and to specification.
I cannot see any contradiction to what I just said. Are you 100% sure that you wouldn't be able to do the same work in 8 hours if you used the extra concentration gained through 2 hours more leisure time? Simply by being more efficient in how you go about your tasks?
Of course it helps a lot if you like what you do "at least half the time". Anything below that would be a sure sign for me to quit immediately.
