"we’re the shortest sleeping primates, yet we pack in the most REM sleep relative to this short duration."
Nothing surprising here. We sleep in cozy comfortable beds in a locked and secure location. Of course we'd can get int REM sleep. Compare to other primates who sleep outside with the critters and predators, deep sleep could mean the end of them.
My understanding is that REM isn't particularly deep. Quick web searches aren't actually defining it as more like light or deep sleep, but many of our systems are more active than other stages[1][2]. Still I don't think it's super surprising that we spend more time in REM since our brains are uniquely complex and we spend a lot of energy on perception and interpretation, which REM seems to help with [1][3]. And I would be willing to be safety and comfort also play a decent role in it as well!
Probably the only primates where a significant portion spent tens of thousands of years isolated up in the Arctic during prehistoric times.
Where the sun doesn't go down every single day anyhow, and you may need to trudge across the tundra routinely for more than 24 hours at a stretch to get almost anywhere, without ever worrying about it at all.
Then they later came down and intermixed with (not) everyone else, so you never know who is going to be less tired than "normal" when they work a 36-hour day.
Nothing surprising here. We sleep in cozy comfortable beds in a locked and secure location. Of course we'd can get int REM sleep. Compare to other primates who sleep outside with the critters and predators, deep sleep could mean the end of them.