Weird. I think that when you've just woken up at night and you're half asleep, you don't know that you're sleepy.
I've woken up several times before school, thinking "wow, I got a lot of sleep, I'm not tired at all! Hooray, getting out of bed is gonna be so eas-"
After that I just pass out. My alarm clock goes off and I can't make myself get out of bed. And afterwards, I remember waking up and not being sleepy.