Knowledge workers have 6 productive hours/day, less than manual laborers  at 8 hours/day, says Sara Robinson:

… knowledge workers actually have fewer good hours in a day than manual laborers do — on average, about six hours, as opposed to eight. It sounds strange, but if you’re a knowledge worker, the truth of this may become clear if you think about your own typical work day. Odds are good that you probably turn out five or six good, productive hours of hard mental work; and then spend the other two or three hours on the job in meetings, answering e-mail, making phone calls and so on. You can stay longer if your boss asks; but after six hours, all he’s really got left is a butt in a chair. Your brain has already clocked out and gone home.

The other thing about knowledge workers is that they’re exquisitely sensitive to even minor sleep loss. Research by the US military has shown that losing just one hour of sleep per night for a week will cause a level of cognitive degradation equivalent to a .10 blood alcohol level. Worse: most people who’ve fallen into this state typically have no idea of just how impaired they are. It’s only when you look at the dramatically lower quality of their output that it shows up. Robinson writes: “If they came to work that drunk, we’d fire them — we’d rightly see them as a manifest risk to our enterprise, our data, our capital equipment, us and themselves. But we don’t think twice about making an equivalent level of sleep deprivation a condition of continued employment.”

And the potential for catastrophic failure can be every bit as high for knowledge workers as it is for laborers. Robinson cites the follow-up investigations on the Exxon Valdez disaster and the Challenger explosion. Both sets of investigators found that severely overworked, overtired decision-makers played significant roles in bringing about these disasters. There’s also a huge body of research on life-threatening errors made by exhausted medical residents, as well as research by the US military on the catastrophic effects of fatigue on the target discrimination abilities of artillery operators. As Robinson dryly notes: “It’s a good thing knowledge workers rarely have to worry about friendly fire.”

Excerpted from “Bring back the 40-hour work week” |Sara Robinson | March 14, 2012 | AlterNet – Salon.com at http://www.salon.com/2012/03/14/bring_back_the_40_hour_work_week/, via https://plus.google.com/104606404307206236075/posts/XAZhqQoU4Tq

Bring back the 40-hour work week - AlterNet - Salon.com