Sleepy, but why...?

Ok, I banked in like 9 hours of sleep last night. Good, solid, deep sleep. The sleep of the just, you know, that sleep I get damn near any night (why, I don't know, I don't question my luck). So why am I tired at work? This is fraking annoying. Especially since I'm not going to be getting to sleep till about 9 Saturday morning. Sunday'll be worse, since I've agreed to be in the back ground of my friends movie, and she's shooting from 9-1 or so on Sunday. But still, why am I tired??? I'm soo happy I've got next weekend off.