4 hours is 4 hours isn't it?

Why do 4 hours in the morning work hours go so much faster than 4 hours in the afternoon? I can't figure this out. I work from 8am-5pm. 8-12 goes by fast, I go to lunch and then 1-5pm seems like an eternity. Does time move at a slower pace in the afternoon? How can this be? I'm busy, I have work to do, but 4 hours seems like days. *sigh* If this would be a single occurance no big deal. It happens often. What gives?