Johann Hari touches on this last one in his excellent book, Lost Connections:
Anywhere in the world where people describe being lonely, they will also—throughout their sleep—experience more of something called ‘micro-awakenings.’ These are small moments you won’t recall when you wake up, but in which you rise a little from your slumber. All other social animals do the same thing when they’re isolated too. The best theory is that you don’t feel safe going to sleep when you’re lonely, because early humans literally weren’t safe if they were sleeping apart from the tribe. You know nobody’s got your back—so your brain won’t let you go into full sleep mode. Measuring these ‘micro-awakenings’ is a good way of measuring loneliness… [A researcher studying the Hutterites in the American Dakotas found] they had barely any. The [Hutterite] community showed the lowest level of loneliness that [the researcher had] seen anywhere in the world. It really stunned [him].
Thanks for taking the initiative to publish this. I offer these resources
A good all-around guide from The Blue Zones
Humans may be hard-wired for segmented sleep
If you are getting poor sleep, you may be lonely
Johann Hari touches on this last one in his excellent book, Lost Connections: