Even when I'm very deliberately making sure there are no distractions (no lights on, no electronics, fan for white noise) and/or I'm exhausted (been awake for >2 days), I find it _very_ difficult to sleep before 3am, and that time will gradually slip back to the point where it's often easier to skip a night's sleep to "resync" my sleep schedule. Probably not the case for every one, but it's difficult to not see that as some innate difference, especially as it has persisted for years across multiple environments.
What is the biological property of 3am to you? Is it a particular time of darkness? Is it a set point of time after you have woken up? What happens when you travel to a different time zone, do you adapt?
As I understand it, it's more of an internal clock that can be shifted by sunrise/sunset light/dark - but there appear to be a biological/inherited component. Hence some mammals have evolved a rhythm more dependent on seasons than day/night, due to polar day/night cycles:
https://www.researchgate.net/publication/41942188_A_Circadia...
Which in turn might imply that humans still being rather young species is tuned to a rather strict day/night cycle, as seen close to the equator.
