That's exactly how I discovered the über-early cycle... I stayed up later and later, one day 8am arrived and I wasn't even slightly tired. I knew I couldn't sustain that sleep cycle (waking at 4pm feels grim) so I pushed my sleep cycle forward, living 26-hour days.
I'm drifting forward (woke up at 7am today), I may be cursed with an inability to sustain a healthy rhythm at one specific time. Not sure yet.
This style of work tends to occassionally push my sleep cycle around in funny ways. After intensive "sprints" it's not uncommon for me to have a week where I wake up at 6pm in the evening. That seems to be my recovery mode then, I don't get anything done in these weeks.
But sometimes, towards the end of such a recovery week, I notice the same thing that you mention. As I slowly adjust my cycle back towards the accepted norm there is often a period where I get up between 2-4am and many of these days have turned out amazingly productive, too.
Sometimes I wish I could just sustain either one of these rhythms (either getting up very early or getting up midday and working late) but for some reason I'm always slipping towards the "recovery state" after a few weeks. I guess that's my body's way of telling me when to stop...