When I stop using alarms and stop forcing myself to go to sleep early, my hours seem to converge to something very similar to yours. I experience the "shifting forward" until I get to 2am-11am, and the shifting stops.

I've noticed the same pattern on roommates and family members as well.

It wasn't always like that.

I fondly the time between school and university, when I was unemployed.

I was awake until 7 and slept until 16. I didn't even party much at that time.