I sincerely believe that circadian rhythms are a part of our biological structure. Just this week I have been watching some videos on plant research and found it interesting that plant life, almost as a whole, actually sleep. Since a plant's life revolves around light & dark cycles, most of their sleep cycles occur at night. People however, (and I don't know this as a fact - I'm only speculating) probably can be influenced by cultural habits (schedules). The last twenty five or so years before my retirement I worked a second shift schedule. (3pm-11pm) When I got home I had to take a couple hours to clean up but mostly relax and unwind. So obviously I didn't get in bed until around 2 or 3 in the morning and usually slept between 8 to 10 hours. Now since my retirement, I go to sleep around 10pm or 11pm and get up around 6am or 7am.
My wife is different altogether. Her father was in Air force intelligence here in the US. She developed and kept his habits of staying up all night and doesn't go to bed until about 6am in the morning. Whether this is encoded in her DNA or not is up to speculation.