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> All humans have a hardwired dip in midafternoon alertness.

In my personal experience, if you don't have a post-lunch carbs crash and you do have air conditioning, the dip doesn't exist.

He doesn't really seem to back up the existence of a biological dip at all that is innate to human rhythms.

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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.

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.

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