We all have the days when you go to the office and don't do anything at all. Facebook. Clean out inbox. Obsess over details like travel plans. Maybe some meta-work such as looking up deadline for conference submissions, renewing library books, volunteering to chair a conference section, speaking to lay people.
And then it gets to be 5 or 6 pm, and I just want to stay until I've done something. But that just punishes myself and it makes me feel anxious and it puts me in a bad position for the next day, and I end up too tired to go to the gym, and it's all downhill from there.
So even though I haven't done anything today, I am going to leave. Tomorrow is another day.
I also write down what I did all day hour by hour, even to fill it in with "facebook", "catch up on dating site email", "catch up on professional email", "answer professional-related email from high school student", "attend lecture". My theory is that it's like the cure for obsessive thoughts. (You don't judge yourself for having them, and you don't try to stop them. You just count them. And before you know it you aren't thinking about that guy who dumped you after a 2 month relationship by losing his phone.)