Humans can be alert and productive for only so many hours a day, differs by person but it is definitely even less then 8 for most everyone. After that something that would take 1 hours in the morning will instead take 4 hours of overtime.
The question is what people could do and people would do. I've had six hour exams and they were killers, same if you watch top chess players after a typical match of ~5 hours so if you're giving it your everything then clearly you don't last eight hours. Do you think people would become super effective if they only worked six hours a day though? Do you think they'll just zone out and mentally recover for the rest of the day? Not just like one day, but every working day? I can't speak for everyone else but I get more done in eight hours than in six or ten hours than in eight. Maybe not quite as much per hour, but it's not like I'm drop dead exhausted when I come home from work. But that's only if I cut down on my leisure time accordingly, if I go from eight hours to six hours to four hours of sleep then yeah productivity goes down the toilet. But that's because I "force" the company to bear that cost, not because I couldn't do twelve hours a day of good work. I just wouldn't have any other life to speak of.