I've been an owl before kids and a lark after my kids were born.
In my experience, it has to do with routine and much nothing else. If I force myself sleep everyday at 10:00 PM and wake up at 5:30 sharp, I find in about a week that my sharpest time is the hours after I wake up and before my kids get out of bed.
When I have the luxury of choice, I stay awake and I find that I fall into focus post 11:00 PM when all is quiet and that deadline a couple of days ahead starts bugging me.
As somebody with a pretty severely delayed circadian rhythm, I believe from my experience that
you actually just have a normal cycle.
With the abundance of electric lighting etc., everyone can be a bit lazy and go a couple of hours later than their normal cycle. But, if you actually were a night-owl, you’d be basically unable to force yourself to sleep at 10pm - at least, not until you become sleep-deprived enough (and not even then sometimes).
This is one of the biggest problems people with actually delayed circadian rhythms have - people with normal rhythms (innocently, but incorrectly) believing that their experience means that it’s just a preference and routine/discipline issue for everyone.
Surely people can hack themselves a routine to wake up early. But like you said, when having the luxury of choice you would likely start drifting back later again. I think that's the innate part people refer to when talking about larks vs night owls.
It's not uncommon to not even be able to force yourself to sleep early, however. I've only ever managed to do that through severe sleed deprivation.
For example, if I have a week when I have to wake up at 7 each morning I will only begin to fall asleep around 22 around Thursday/Friday. Up until that I've just fought my natural cycle: I can not force myself to fall asleep but I can force myself to get up early. So, I get less and less sleep until I practically fall asleep sitting. Now, if I continued the 22-07 rhythm I'd eventually stop being a zombie and become productive again (done that, too), but as soon as I finally have the luxury of choice again I'm very soon back at going to sleep after midnight, on the next day or so at the latest.
I was about to report the same personal experience, so that's twice the amount of anecdotal evidence in favor of the environment/habits and against inner causes.
In my experience, it has to do with routine and much nothing else. If I force myself sleep everyday at 10:00 PM and wake up at 5:30 sharp, I find in about a week that my sharpest time is the hours after I wake up and before my kids get out of bed.
When I have the luxury of choice, I stay awake and I find that I fall into focus post 11:00 PM when all is quiet and that deadline a couple of days ahead starts bugging me.