I've noticed that when I even partially cut refined sugars from my diet, I get through the day with a much clearer mind, particularly in the realm of executive/self-disciplining functions. It's to the point that I'm noticeably more productive at work without refined sugar.

ISTM that for me in the short run it's the other way round, but that's probably got to do with the fact that most of my sources of refined sugars are sources of caffeine and water as well.