I'm in the middle of it because of a Hacker News recommendation, and let me pass on the recommendation. It's great so far :)
For example, I think most of us now know that napping isn't for the "lazy" but it's nice to see the scientific reasoning for it. The author Walker is a great sleep advocate.
The 35 year old theory seems equally as likely as the threat-simulation theory. What has made threat-simulation supplant other theories on this topic?