I think these types of studies are all going to be fatally flawed with a bunch of very-hard-to-untangle factors: since "40 hours" (or more) is the "expected norm" then if you're working less you've either fought incredibly hard to do so (which I personally have), you have way more freedom to choose working hours than most people or you're "forced" to work less (or more) for some reason.

e.g. my first thought about the "increased hypertension in workers reporting < 7 hours" (a paper I haven't read, mind) is that they're stressed about finances because they're working part-time (or the "hours" was averaged over the year or something, and they spent some time without a job).

Now, if you've chosen to work many more hours then it seems intuitively obvious that you'll be more productive. There's also the hard-to-study factor of what you're working on. For example, many free-software developers are doing their free-software-developing in their "non-work" hours -- so they might be "working" (in the sense of producing code) 80+ hours a week, but will likely only get "counted" as whatever their day job was. (If there are studies that account for "what you do with your other time" against productivity, I'd be very keen to read them).

In a similar vein -- and as John said in the original post -- if you're "stuck" on something (e.g. can't do any more "design the hard algorithm" thinking) and switch to something "boring" that still needs to be done (e.g. make the Makefile suck less) that's still "more productive". But, as others have pointed out, many developers don't have that freedom -- they have to keep pounding their face against whatever bug they got assigned. And if you're stuck, but have to keep working on it, I'd bet a lot that your productivity goes to shit.

So, all that said, I personally believe (note: not scientific! ;) that we'd be far better off to have shorter expected/forced "working weeks". And I don't mean that everyone should be forced to work less! This may (or may not!) have a "productivity" impact for employers but if you looked at individuals' "productivity" (including work and "non-work" parts) I think it would go way up -- precisely because this would give the vast majority of people way more choice about what they get to work on. This -- I completely unscientifically presume -- would also have a large positive effect on people's reported happiness.

That is, they'd be spending less time smashing their face on that bug at work and get more time to do whatever else they like to do -- free-software, painting, writing, building things, etc.

In any case, I know that for me personally being forced to work 40+ hours a week on the same thing is completely toxic to my creativity and productivity at programming and I've worked very hard to get paid less (to work less). I can also tell you that nearly every co-worker or friend I've told of my myriad different arrangements to achieve this have wanted to do the same (but feel they "can't"). The only exception seems to be when people realize the bit about "paid less" ;) and can't do (or don't want) that. Interestingly, I still have spent a bunch of my increased leisure time writing software -- but for myself, or free-software instead of "for work". Of course, I've also spent a bunch of it doing things for pure enjoyment.

Now, of course, that's all just anecdotal. But, for me, less "must work on X for $" time means a lot more happiness and a lot more "productivity" (if you look at everything I do with my time).