Ok, there’s just so many facts contrary to all normal experiences, so I’d like to see some sources. For example, I can easily work twelve hours per day while painting the facades of my home, without being mentally fatigued, only bored. But my body is surely sore after climbing up and down stairs all day. On the other hand, I’m hardly capable of doing six hours of really focused mental work per day.
Manual laborers are people no less than accountants or mathematicians.