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.
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.