We often cite this fascinating (and very readable) experience sample study out of Harvard Psychology.
Killingsworth & Gilbert (2010) A Wandering Mind is an Unhappy Mind.pdf
Matthew Killingsworth collected a lot of data and makes two really interesting and important assertions:
1) People's minds wander about half the time
2) We are generally happier if we are paying attention to the task we're doing, regardless of what that task might be. Doing a desirable task does affect our happiness, but paying attention matters more.