I managed to get myself into the lark schedule three or four times for various periods of time (2-3 weeks minimum, about 6-7 months in the longest streak) but it took a lot of energy to keep myself there. I had to get up very early even during weekends, and push myself to go to sleep around 10 PM, because a single hour off my regular sleep hour was enough to kick my schedule into chaos again. I wasn't drowsy during the day, but I wasn't too productive, nor too energetic, either, and overall my productivity plummeted.

I eventually gave it up. It reverted to more normal hours when I moved in with my girlfriend, who wakes up a bit earlier. Nowadays, I can typically wake up around 10:30 AM without an alarm (and would generally fall asleep around 4 AM), which is a lot better than before when, left to my own devices, I'd wake up around noon. During weekdays, I wake up around 08:30 (fortunately, my workplace is quite close to home -- it's a twenty minute walk); I need the alarm clock for that, but except for Fridays, I'm not too exhausted.