I have a pretty annoying sleep disorder called delayed sleep phase disorder (DSPD), diagnosed by multiple clinicians. I have impeccable sleep hygiene, quit caffeine entirely, tried lots of OTC and prescription sleep drugs, light therapy, every kind of sleep supplement you've heard of and probably a few you haven't, and I still revert to going to bed around 5am and waking between noon and 1:30pm.
In the times I maintained a "normal" early-bird schedule, I actually consistently fall into a deep depression.
Our circadian rhythms are pretty powerful regulators -- not just of sleep, but of our hormones and regularity (in the poop sense) too. So misalignment wreaks havoc on our bodies in various ways, even if we're able to get enough sleep.
Anyway for some people they're more flexible and trainable than others. I'm genuinely happy for your case, but I mainly wanted to point out that not everyone will have the same results.