In short, Henry Ford did experiments in his factory and decided that 40h/week was optimal. Unfortunately, this figure has stuck, even though newer research indicates that for knowledge work and creative work, less than 40 is more appropriate. (35-40 suggested by these slides: http://www.lostgarden.com/2008/09/rules-of-productivity-pres... )
In short, Henry Ford did experiments in his factory and decided that 40h/week was optimal. Unfortunately, this figure has stuck, even though newer research indicates that for knowledge work and creative work, less than 40 is more appropriate. (35-40 suggested by these slides: http://www.lostgarden.com/2008/09/rules-of-productivity-pres... )