If one does very intellectual work, 8 hours of solid work are pretty hard to achieve. Hardy said in that "four hours creative work a day is about the limit for

If one does very intellectual work, 8 hours of solid work are pretty hard to achieve. Hardy said in that "four hours creative work a day is about the limit for a mathematician" in A Mathematicians Apology. I think for engineering work, one might be able to stretch it a bit further. But factoring in breaks, I don't think it is sustainable to go beyond 7 or 7.5 hours.