It seems that 40 hours is basically optimal for turn of the 20th century factory work. I've seen several references[0] that the optimum for knowledge work is 3

It seems that 40 hours is basically optimal for turn of the 20th century factory work. I've seen several references[0] that the optimum for knowledge work is 35 hours, but I'd LOVE a detailed citation. While you are correct that people vary, many managers seem extremely ignorant of 200 years of research on this topic and start from grossly wrong initial conditions (e.g., "40 hours is the bare minimum and should be considered slacking: a professional works until the job is done and we really expect 50 hours; we prefer passionate people, which starts at around 55 hours"). It would be far better for almost everyone if the typical standard expectation were based on a typical standard human, not on outliers (e.g., people who carry the DEC2 mutation and need only 6 hours sleep). Look at it another way: the 75% of the workforce who is average would stop burning themselves out to the point of negative productivity, and supermen like yourself would be truly recognized for your outstanding contributions, rather than just "meh, meets expectations". [0] e.g. http://www.slideshare.net/flowtown/rules-of-productivity-2756161 http://www.slideshare.net/flowtown/rules-of-productivity-275...