Well researched and full of powerful anecdotes. A couple of problems stop me from enthusiastically recommend it though:
1. It doesn't live up to the tile: the book is mostly a collection of facts and interesting research results and it fails to provide any kind of overarching theory of why we sleep.
2. The main tool used in the book is the scare tactic: "here's what happens if sleep quality is compromised". It works, and it might even be the right approach in many cases (schools starting before 9am is simply barbaric). The problem is, for someone going though sleep troubles, this can dramatically make things worse. It took me ~6months to recover from this book.