I just read this book last month on a friend's recommendation. I don't want to pile on whatever anyone else said, but I had noticed a contradiction in Chapter 6 that I found to be very distracting, and I'd love to hear from anyone else who knows the facts behind the subject.

First, in Chapter 3, he introduces "light NREM sleep" as stage-2 NREM, while "deep NREM sleep" is stage 3-4 NREM.

Then, throughout most of Chapter 6, he says that you get a huge "memory restoration" benefit from light stage-2 NREM, due to sleep spindles:

* "The memory refreshment was related to lighter, stage 2 NREM sleep, and specifically the [...] sleep spindles."

* "The more sleep spindles obtained during sleep, the greater the restoration of learning."

* "The concentration of NREM sleep spindles" happen in stage-2 NREM, and you get the most of this type of sleep after 6 hrs of sleeping.

* In the first 6 hrs, you've gotten all your deep NREM sleep: "we obtain most of our deep NREM sleep early in the night, and much of our REM sleep (and lighter NREM sleep) late in the night."

* "Sleep six hours or less and you are shortchanging the brain of a learning restoration benefit that is normally performed by sleep spindles."

* Furthermore, stage 2 NREM is important for other kinds of restorational benefit: "The increases in speed and accuracy, underpinned by efficient automaticity, were directly related to the amount of stage 2 NREM, especially in the last two hours of an eight-hour night of sleep [...]. Indeed, it was the number of those wonderful sleep spindles in the last two hours of the late morning—the time of night with the richest spindle bursts of brainwave activity—that were linked with the offline memory boost."

So he firmly establishes this point about light NREM sleep throughout Chapter 6.

However, but there's a section ("SLEEP-THE-NIGHT-AFTER LEARNING") where he makes a big deal about the distinction between deep NREM and light NREM sleep, calling it a "battle royal":

> You will recall from chapter 3 that we obtain most of our deep NREM sleep early in the night, and much of our REM sleep (and lighter NREM sleep) late in the night.
> After having learned lists of facts, researchers allowed participants the opportunity to sleep only for the first half of the night or only for the second half of the night. In this way, both experimental groups obtained the same total amount of sleep (albeit short), yet the former group’s sleep was rich in deep NREM, and the latter was dominated instead by REM.
> The stage was set for a battle royal between the two types of sleep. The question: Which sleep period would confer a greater memory savings benefit—that filled with deep NREM, or that packed with abundant REM sleep?
> For fact-based, textbook-like memory, the result was clear. It was early-night sleep, rich in deep NREM, that won out in terms of providing superior memory retention savings relative to late-night, REM-rich sleep.

This doesn't necessarily contradict the idea that the spindles during light NREM sleep are restorative, but it says light NREM is less effective than deep NREM, and then he doesn't cite any more research on deep NREM's memory benefits. In chapter 7 he focuses on how deep NREM sleep has other benefits. The author does mention "deep NREM sleep" as important for cognitive benefits later in the book when it seems he meant to say "light NREM sleep".

Also, to say they allowed participants to sleep "only for the second half of the night" is strange, and I don't know what that means. Looking at the original study
https://www.jstor.org/stable/1414040 it looks like there were ONLY TWO subjects, and it's not clear to me that this study was described accurately by Walker.

If this point was already discussed elsewhere, I'm not aware of it, and would appreciate a link.