Wendy, you have an office in an academic building. You are exhausted. Shut your door, and put your head down or lean back in your chair for 20 minutes. Seriously, you will feel so much better. At my old job when p/g with DS, I napped on the train on the way in and on the way home, and for 20 minutes in the afternoon. I've only had to do it once here, plus leave early once to go home and rest before DS got there!