Unemployment solved: 20-hour work week

While watching yet another presidential candidate promise how he would stimulate the economy to create more jobs, I found myself wondering why nobody is talking about an obvious solution: restricting the supply of labor by cutting the working hours.
 
Given the current productivity levels and 5 billion people of working age on the planet, it does not seem possible to give everyone a 40-hours/week job doing something useful.  There is a precedent for reducing working hours.  We went from 70 or more to 40 a while ago.  We also eliminated child labor.  Was it a coincidence that real wages went up and the middle class got created since then? 

Why is 20-hour work week a bad idea, or a good one?

>>8
A cultural shift would be good.  This could be accomplished by adding a new commandment to all mainstream religions: Thou shall not labor for wages for more than 20 hours a week!

To help any non-believers get on board, the new working hours would also be recognized by the United Nations as a basic human right, and any countries violating it would be subject to severe sanctions starting with an economic embargo all the way to regime change.