On: Humans used to sleep in two shifts

[sneak]

I'm curious about them; what are some titles?

[enraged_camel → sneak]

I don't know about books but this isn't the first time people have written about this. Doing a quick search on "between the sleeps" yields articles from early 2000s and 2010s.

[garmaine → enraged_camel]

And it’s one of those articles I read which had the tidbit about the books, and even some choice quotes. Not sure which article it was, but google should find it for you.

[Jiocus → sneak]

You could buy the book that sourced those original titles to find out. Authors should be compensated for their work.