There's been a lot of research done into "ego depletion" - basically, 100+ studies have found evidence that the ability to focus and make yourself get things done is an exhaustible resource, and then recent studies by Carol Dweck have indicated that your abilities to do so, in some sense, depend upon your mindset going in and whether you embrace the task to begin with. The logical place to answer this would be with an fMRI study, and the first such study was just published a week and a half ago. The abstract indicates they found something, but the article itself is paywalled:
http://scan.oxfordjournals.org/content/early/2012/09/20/scan...