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?

>>32

The selling price of a widget (or service) includes many other components besides the wages of the workers.  The cost of some of these (i.e. buildings, equipment, etc.) might be fixed.  Though read my previous post about debt adjustment that would go with the reduction in working hours.
 
Other components of the selling price are, shall we say, rather elastic.  These include a markup due to monopolistic market share, dividends distributed to shareholders, the exorbitant salaries of certain employees (i.e. CEOs and top management).

Any work past 20 hours would be considered overtime.