On days I wake up naturally, early in the morning, I am 100x more productive than a typical day. The problem is keeping that consistent seems to be really difficult for me, possibly depending on a bunch of factors, like stress levels, exercise, sleep quality. It might even depend on the season, with cooler months somehow being better for me.

I noticed the very same. I often feel fresh just the moment I open my eyes without an alarm. Tho it's also true that in some occasions I wake up and realize you need 30-90min more so and I try to dig back in :) All this is altered by any alcohol consumption the night before - proportionally to the amount.