I think a version of the second and third possibilities is probably true.
For DEC2 and GRM1, the mutation decreases the ability of the gene to block wakefulness (DEC2) or promote tiredness (GRM1), and loss-of-function mutations seem easier to create. And indeed, I think one of the papers mentioned that there are other mutations in the DEC2 gene that correlate with reduced sleep, and GRM1 already has two identified short-sleeper mutations. Therefore, I expect that similar mutations have appeared in the past, but didn’t catch on.
Why would evolution have rejected them? I suspect that, as you say, 10,000+ years ago (perhaps even 1000 years ago), being awake an extra two hours before dawn brought little benefit and basically just meant that you wasted calories; the tradeoff only became worthwhile very recently, in evolutionary terms. Perhaps there were also effects like “people had a higher parasite load, and/or were constantly scraping their skin or stepping on sharp objects, and thus had a higher constant need for some type of repair that the mutations don’t accelerate”.
I think a version of the second and third possibilities is probably true.
For DEC2 and GRM1, the mutation decreases the ability of the gene to block wakefulness (DEC2) or promote tiredness (GRM1), and loss-of-function mutations seem easier to create. And indeed, I think one of the papers mentioned that there are other mutations in the DEC2 gene that correlate with reduced sleep, and GRM1 already has two identified short-sleeper mutations. Therefore, I expect that similar mutations have appeared in the past, but didn’t catch on.
Why would evolution have rejected them? I suspect that, as you say, 10,000+ years ago (perhaps even 1000 years ago), being awake an extra two hours before dawn brought little benefit and basically just meant that you wasted calories; the tradeoff only became worthwhile very recently, in evolutionary terms. Perhaps there were also effects like “people had a higher parasite load, and/or were constantly scraping their skin or stepping on sharp objects, and thus had a higher constant need for some type of repair that the mutations don’t accelerate”.