My ex was a night owl. He never stopped being a night owl in twenty-two years of military service.

Most people are night owls. So I imagine he's not the only one who was a night owl and had a successful military career.

My father was a morning person and spent 26.5 years in the army. At one point, they had him working in the kitchen because you had to be there at 2am to have breakfast ready by 5am or whatever and most soldiers could not cope with it. I think he did that for something like a year.

There shouldn't be any self-selection if these people don't exist. We're being told that "adolescents" all require sleeping late, thus school should start later for all students.

Or maybe the more structured environment of the military enforces early bedtimes vs. lax family life where parents zone out on smartphones while their kids are left to their own devices (pun intended) to enforce their own bedtimes.

Correct. It’s all discipline. When the school has late start, do the kids go to bed at the same time and get more rest? Nope. They stay up later.

You sign up voluntarily for the military in my lifetime. No so much for school. People tend to submit more readily when they aren't being compelled to do something. Even if they don't end up liking it, there's a tendency to act in accordance with a choice you made.