Wow, if one can increase productivity by reducing hours from 90 to 60, then again (as you say) from 60 to 40, then what really is the optimal hours worked per week? I would think it largely depends on the task at hand - manual labor or academic. I know when I framed houses I could do 6x10s and was wasted beyond that and useless, but mental work can be exhausting also. I know with practice one can build up the ability to work longer hours physically and I suspect with training you could do the same mentally. The bigger question, however, is people are expected to have a life outside of work, too. Personally, I'd like to see 35 hours as you suggest as the law.