How to tell when you're too tired to work any more

You are making notes on a book and write this sentence: "Baby vampires set to work on wrappers." The book is not about baby vampires, or any kind of vampires, for that matter.

The automatic utility on your backup hard drive kicks in and you think it is some song by a new artist that Pandora thinks you might like.

You are not longer sure whether the lights on your desk are on. They are. Your eyes have closed.

The cat stretches and gets down from the top of the filing cabinet where she keeps you company all day to go and shed fur on the chair where she sleeps at night.