Matthew Walker's excerpt on biphasic sleep: https://delanceyplace.com/view-archives.php?p=3524 TL;DR: There's evidence of cultures untouched by electricity sleeping 7 hours at night with a 30-60min afternoon nap, which he calls biphasic sleep, with some tribes switching to monophasic in cooler months. All humans have a hardwired dip in midafternoon alertness.

> 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.

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.

I believe the original parent is refering to the idea that people naturally wake up and do stuff in the middle of the night.

Yes, but I couldn't find where Matthew Walker says that.

True, I was referring to waking up in the middle of the night for an hour or two and then going back to sleep.