I really loved this book and it's having a big impact on my life. Very useful.
That said, his writing style can be questionable. I expected a book where everything is well cited and described. Rock solid facts and info on sleep from this PhD. Instead, there are lots of claims made (with flowery and strong language), but the citation is relatively infrequent, and sometimes I wanted one to go check the source.
Sometimes, info presented seems to conflict with what came before. He says that Light NREM sleep (stage 2) is primarily responsible for "sleep spindles" and memory transfer from short term to long term, and then later he drops a generic statement about how stages 3 and 4 are primarily responsible for this activity. He says "For these reasons, melatonin is not a powerful sleeping aid in and of itself..." after describing how melatonin kicks off the processes of your body's sleep, but doesn't drive the sleep itself; then, later in the book, he describes how viewing screens late at night (which most of us do) delays the release of melatonin and your body isn't ready for sleep when you finally go to bed. The logical conclusion of this is the melatonin would be a powerful sleep aid for those who continue to use screens at night. I found myself constantly feeling slight contradictions and undersupported statements in this book and it reduced the credibility of the entire book by just a bit, hence the four stars.
That said, read it. Most people know almost nothing about sleep and this changed by understanding of it and provided many valuable and enjoyable facts. I'm going to try biphasic sleep as I'm lucky enough to control my work schedule... plan is to sleep 10-5am and have a 90min Siesta in the afternoon to help solidify mornings learnings and clear the brain for more that afternoon.
UPDATE: I went through and reviewed key passages, a habit of mine a couple months after finishing a book, and feel even more strongly about the many claims unsabstantiated by research or evidence. Another example is the alarm clock section. He describes a reasonable story, how we are "shocking our nervous system repeatedly", and makes it sound like an incredibly damaging thing with his intense language, and leaves it at that. Is it damaging? Has anyone looked into this? Has Dr. Walker? It just makes it hard to believe a lot of this book.
Another example: "A study across four US companies... showed a net capital loss of $54M. Ask any board of directors whether they would like to correct a single problem fleecing their company of $50M and the response will be rapid and unanimous." It's $50M across four companies, not one, but he then frames the punch line with it being about one company, which is basically using a 4X exaggertion of his supporting data to make his conclusion more punchy. You might say that is minor, but it is how he writes this entire book, which adds up to dubious claims and, for me, overall distrust of his statements. This is really too bad, because I believe much of the points he makes are valid, but I wish I could trust his statements more.