The theory here is that not only are you at 20% for the last hour: you will be at 90% at the beginning of the second day, 80% on the third and so on. Of course

The theory here is that not only are you at 20% for the last hour: you will be at 90% at the beginning of the second day, 80% on the third and so on. Of course the percentages are totally made up, but it is obvious that there is an amount of work from which no single instance of rest can cause a complete recovery. If the results of such unrest are cumulative, then there must exist some amount of work that is less productive than working less for any kind of sustained endeavor. The problem is we have no real way of measuring personal productivity, so we have no real way to find where the optimal point is for any individual. Society as a whole has chosen 40 weekly hours as a standard for mostly orthogonal reasons. In aggregate, the number is probably wrong. But it seems crazy to me to argue that there's no (sizable) fragment of the population that would not be as productive, or more so, with less than the 8 hours they work now.