There have been some studies, but I dont have the references at hand. Fasting can regenerate the entire immune system for one, it can also assist in accelerated tissue repair. Anecdotally I can tell you I've been far more productive today than most days because I'm being more conservative with how I expend my energy therefore didnt waste time on things like reddit and FB, and because my body isnt busy both worrying about my next meal or digesting it, I've had much greater clarity of thought which has been great for focus.

Edit: sp and grammar