I sleep at 3-4 AM most of the time, sometimes 1 if I'm tired...and get up at 8.
Guess I'm not old.
Oh wait...18..heh
But nah, the reason you got tired is because you probably weren't used to it. Although if you're over 40, you're slowly starting to get old, differs on how well you kept your body when you were younger I guess.
Biological age is what matters, not the ''official'' age.