Well if you've got to sow your field before the snow comes then it's not real helpful to have the guy next to you telling you that after 8 hours you're producing diminishing returns and should call it quits. That's going to be interesting advice next spring when you don't have a crop.
The diminishing returns only applies to creative work, not physical labor. (Yes there is diminishing returns there too but clearly you're still getting additional useful work done)
