you know you're tired when

You've trudged back to your office after caring for patients in three separate locations over thirteen hours.  You just want to finish your notes and collapse into bed.  You don't even care what you listen to so you put on Tea for the Tillerman even though you're already exhausted and a little Cat "Ambien" Stevens might put you over the edge to sleepyland.  And then you're so vigorously writing notes that you don't realize that you've been listening to "Where Do the Children Play?" over and over again for 56 straight minutes because the repeat button was actually clicked.

Yep, that's when you know you're tired.

Or that it's time to rethink things.