Darrell Norton has the takeaway from a study regarding the number of hours an employee works per week and productivity. Original study here.
As someone who until recently quite often put in 60 - 80 hour weeks and am now limiting myself to 40, I can tell you that without a doubt I am able to produce higher quality work and my cognitive function, stress level, and overall health are much much greater.
I also think that this is one of the most underrated and underhyped tenets of eXtreme Programming. Pair Programming, Continuous Integration, and Test Driven Development get all the glory, but buried down in there is an encouragement toward Limiting Work Hours in order to achieve more Energized Work. This, along with the overall insistence on a holistic approach to valuing software development and developers as human beings, is one of the things I find so attractive about XP on a personal level. It's ashame that it gets so overshadowed by the "big boys".
In light of EA-gate, I think it's time for this practice to start getting the A-list attention it deserves.