You know you're really tired when someone asks you what 8 x 4 is and you can't figure it out.
This also reminds me of a time when I hadn't gotten any sleep for 36 hours and as a result it took me 3 hours to do half of a "Very Easy" Sudoku, one that under normal circumstances I would have finished in 15 minutes tops.
Ah, the joys and perils of not getting enough quality sleep.
"And somewhere in the northwoods darkness a creature walks upright. And the best advice you may ever get is: Don't go out at night..."