1. The text in the document you’re supposed to be proofreading starts swimming in front of your eyes
2. You’re afraid to blink in case you can’t manage to open your eyes again
3. Your head feels like it’s stuffed with rocks which are dragging it down towards the desk while at the same time feeling so light it may just float away
4. You’re freezing to death despite the fact that it’s not even cold because all the energy you should be using for warmth is being expended on staying awake
5. Your eyes feel sore and you’re convinced the bags under them stretch all the way down to your nose
By the way, can you get repetitive strain injury from rolling the mouse wheel? Just wondering…
Confuzzledom 
Ooh, I can add another to this – all sense of time is distorted. A second feels like an hour, and sometimes an hour is like a second only you can’t remember what happened in the lost time!
Oh, and the marrow in the bones of my legs actually aches when I’m really tired. Joy!
I actually wanted to add one about having no idea what day it was but I was so tired I forgot it.
My throat hurts when I’m tired.
I’m sure you can get RSI from mouse-wheel-rolling … which is why I so hate websites and e-newsletters that just go on and on and on and on…
One of my tiredness symptoms is thinking I have asked a question of someone when I have only asked it “in my head” and not out loud … and then getting quite tetchy with the other person for not replying.
UGH. I love that “can’t keep my eyes open” feeling. ESPECIALLY when i’m in a work meeting. Bah.