My natural sleeping rhythms make me a "night person". During summer vacation, absent some reason not to, my usual sleeping time is 2am to 10am.
Staying up late at night over Christmas break, though, has caused me to pay a price. Even though I tried to go to bed and wake up at reasonable hours this week, I've still felt like death-warmed-over at school. Today I was at least awake enough to function reasonably well; I'll probably be completely back in the groove tomorrow.