[medell]

Isn't he claiming the opposite? (But I don't have the book to dig into the sources):

This brief descent from high-degree wakefulness to low-level alert­ness reflects an innate drive to be asleep and napping in the afternoon, and not working. It appears to be a normal part of the daily rhythm of life.

[Izkata]

I've always understood it to be related to the hottest part of the day, not so much a post-lunch thing. Probably why GP mentioned air conditioning preventing it.