Paraphrased from the book - basically once you get passed the number of REM cycles needed all of the sleep after that is worthless and ends up just making you groggy. In other words the idea of "catching up on sleep" is BS.
As I noted in this thread - I don't think 100% of what author communicates in the book is factual or conclusive, but he does a great job of collecting multiple perspectives and studies to combine it into one thought provoking read. Like any book that has an agenda/angle, a healthy dose of skepticism is always required.