A colleague recently ask for evidence that 40 hours per week is an optimal work schedule. Here is a document that talks about past studies on “crunch time”, working more than 40 hours a week. It shows that after 4 weeks of crunch time work, productivity declines below the productivity teams had in prior weeks working 40 hours a week.
This result was most recently repeated by High Moon game developers, in 2008, see Snap, Crackle, Crunch Time. However, this just confirms that game development replicates what others have shown for different industries, including Henry Ford about 100 years ago.
See Rules of Productivity for some nice summary slides.