As others have said it likely just depends on the ratio of busy work to complex mental work they're doing. I had a stint of several months so far this year whe

As others have said it likely just depends on the ratio of busy work to complex mental work they're doing. I had a stint of several months so far this year where I was putting in 100+ hours/week of manual labor at a day job and then coming home to do a couple hours of programming before sleep. Hell, I even started learning Thai during my half hour breaks. I didn't find myself having any serious difficulty maintaining that lifestyle after having adapted to it. I honestly felt great but couldn't shake the thought that I was probably abusing my body to the point where I'd pay for it later. Oh and yes, some coworkers liked to toss out the "superhuman" quip but I don't think I was doing anything others my age couldn't do. Youth and determination make for a powerful combination.