> Humans are built for constant labor. We're the endurance-running champions of the animal kingdom.
I mean, sure, we can do that. But most of our ape relatives lay about all day socializing and taking advantage of their social cohesiveness to hunt-and-gather very quickly (and then get back to lazing around.) They have many capabilities, but rarely use them, because social organization is just so OP compared to the defenses of most of the animal kingdom.
The big strong apes (e.g. gorillas) don't use their strength for anything day-to-day; they just use it in occasional tribal territorial disputes and social-hierarchy disputes, which don't take up all that much of their time, all-said. Certainly not enough to wear them out by the end of the day, every day.
I don't see any reason that our sleeping habits would have gone through a big evolutionary shift relative to all the other apes; so if they can—and do—sleep just fine without exhausting themselves, we very likely can as well. If we can't, it's very likely not "lack of exhaustion" that's the cause.
IMHO, exhaustion seems to just be a hack, overriding the concerns of some other part of us that thinks it important for us to stay awake; rather than the proper resolution of that mechanism's concerns.