Right now, my normal work day starts with my waking up 5am and I feel like shit most of the day regardless of how long I've been doing it and how disciplined I am about getting to bed.
My natural cycle has me waking up at around 10am and falling asleep at around 2am.
Guess how many days it takes for me to readjust back to my natural cycle when I have some time off and turn off the alarm clock?
The next day. I'll crash early like normal the night before and sleep through an extra 5 hours to wake up almost on the nose at 10am then fall asleep the next night at around 2am.
This happens when traveling overseas too. Within one or two days I'm back on the 2-10 sleep schedule.
And I feel awesome when I'm on that schedule. Even better, I avoid most traffic on those days.