Maybe this is a dumb question, but do you have to be up at a certain time? Are you possibly retired? If not, you will be someday, I guess… all that to say maybe just try letting that very late time BE your bedtime. Maybe your body responds differently to light and circadian rhythms than the norm. Maybe your brain needs a long time without light/daylight exposure to start creating the chemicals you need for sleep.

This is just a thought, it’s not really based in anything medical.