Having worked longer than 80 hour weeks for years at a time at full productivity I know it is possible for at least some people. The issue for me was not one of fatigue, but of a slow corruption of my perception of reality. The longer I worked at this pace the worse my ability to differentiate important from unimportant became and I made some very "interesting" decisions during that time.

I still work more 40 hours a week, but nowhere near the level I did in the past.