Also, there is an assumption that everyone can be productive for any length of time without some form of intermission to break it up. What if most people need a 5-10 minute break for every 30 minutes work? You might not be able to get around that by just making the daily or weekly work hours shorter. You'd just be applying the productivity factor to fewer total hours.

I doubt we've found the optimal work to rest ratio, but a lot of these assumptions go unchallenged in these discussions.