You know you're tired when...

You keep re-reading the same page of an article that you're really interesting in reading.

You're too tired to go two doors down and recover your lab book that you left yesterday to work on the writeups.

You're cold so you're sitting there bundled up trying to keep warm and everyone's looking at you funny.

The one thing that keeps popping in your head is the comforting thought of laying in your bed.

Three cups of coffee feels like you just poured it down the drain.

The first thing you pull out of the closet to wear to work are a new pair of pajamas.

A pile of cat puke at the opposite end of the bed from where you sleep is not a deterrant to going to bed.

These are the only things you can come up with.