This is true as long as there aren't activities where you can have negative productivity. e.g. In software, in my 80th hour, I could quite easily write some co

This is true as long as there aren't activities where you can have negative productivity. e.g. In software, in my 80th hour, I could quite easily write some code that was bad, that takes longer than an hour to work out is bad. Code that wouldn't have been written wrong if I had been thinking clearly. Hence is net negative in terms of productivity. (not saying that the point at which productivity goes negative at the 80h mark however.