Long days of work are often far more productive for me. It usually takes me a couple of hours to get the entirety of the problem/solution in my head. I might accomplish five or ten times what I would if that 18 hours was split across three days.

That's a different situation than what I am referring to. You're talking more about being in flow. I'm talking about poorly run projects where it becomes a requirement to work long hours and weekends. It simply isn't a sustainable way to work.