I think the OP is talking about knowledge workers specifically. If you're a security guard, I think it's obvious you could do your job for 100 hrs with very little drop off in performance.
Don't think that's true, watchstanding positions are amongst the most mind numbing jobs that actually require your full attention. I stood watch in the Navy as officer of the deck in port, and I can truly say that 6 hour shift was torture for the mind, much worse than coding 6 hours.
To a point I guess, but I imagine the drop off in performance from 2 hours to 8 hours of standing watch is much larger than the one from 8 to 20 hours.
From 8 to 20 hours watchstanding, you're basically not watchstanding. It went like this:
Hour 1: Fine
Hour 2: Walk around a bit, then fine
Hour 3-5: Jesus, I never should have joined the Navy
Hour 6: I want to pull out my 9mm and use it on myself
Hours past 6: I am functionally useless
Some jobs that aren't knowledge-jobs are hard on your mind, too. The reason security/watchstanding is so hard is that you have to be alert at all times while doing, effectively, jack squat. It isn't a much different level of alertness than I am in while programming, now that I think about it carefully.
Granted I've only met 5 security guards in my life. But not a single one gave the job their full attention. All basically saw the job as a warm body position.