The importance of sleep is covered in chapter 15 and the retarding effect of fatigue and the mental "second-wind" are covered in chapter 13.
I read this book (from Manybooks.net; thanks, Patricia, for converting it for MR) last fall and was quite impressed with how current it remains. I wish I had read it and taken it seriously when I was young. I was particularly affected by Kitson's comments about the neurological importance of habits to thought processes. Sounds perfectly obvious but the obvious sometimes isn't paid much attention to - certainly not by me!