Personally, what works best for me is a bit of a reverse. I will break my day up into 3 hour chunks but I plan my breaks more than those work chunks. I know what has to be done in those chunks. Focussing on each 20 min break means I can get all of my hackernews reading one, quick scan of my inbox and social media. It could also be a walk and a call with a friend or a cup cake :)