In one the latter chapters, he has a rule about "summoning your own enthusiasm". The primary observation in that part of the chapter was that it's really hard to truly exhaust someone physically, and even harder to exhaust them mentally, if they're doing something they actually care about. He gave some examples, one involving special forces people, and concluded that the primary source of tiredness for most people is lack of motivation - lack of interest, or not being able to feel there's a goal to all the work one is doing.
While my opinion is that Carnegie was exaggerating a bit here, I won't deny that this rings very true to my experience. As a programmer, I don't get physically taxed by work, and I very rarely get mental exercise too (most programming on regular web jobs is just plain tedium). But I do feel really tired on the job very quickly, and - back when I was a regular employee - the feeling used to evaporate almost immediately when I clocked out for the day.
I can even give an example from yesterday - lots of work and a visiting a doctor made me really tired by the evening, but I decided to sit down and write some code for a side project. My tiredness was gone immediately, and after 2 hours of cranking out code, I had to force myself to go to sleep, in order to have any energy for the next day.