Consider this, imagine someone working every single waking minute at their job. Clearly they will not be very happy and after a time not very productive. So there exists some optimum level of productivity between 0 and 18 hours of work per day. The big question is whether it is closer to the 0 or closer to the 18. Currently there is some good evidence for it being in the 3-6 hour range(I don't have the studies on hand, maybe someone else can chip in here). Now that is of actual productive work and not "work"(where you are just visible in the office or sitting at a meeting table).