I suspect that when you're dealing with the brain there's little difference between physical and psychological: if the brain's hitting a wall physically it will unconsciously invent some explanation. I suspect that's what's happening with these startup founders who think they are working 18 hour days; anecdotally, when I've pushed myself to work 12-14 hour days, I haven't felt unproductive, I suspect my brain just shut down so that it couldn't register all the time I was wasting. If I look at my actual changelist history, I'd find that the first 3 changes of the day would take a total of about 3-4 hours and then the last one would take 7+.
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:
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...