I wonder how long this productivity increase lasts. From what little I know on psychology, people’s productivity and motivation increase after they get some benefit - a raise, a promotion or a bonus. Then after 6-12 months, their productivity goes back down. Could the same effect be seen with cutting down the work week from 5 to 4 days - and that all these studies were shorter than 1-2 years to fully evaluate the productivity gains?

To better explain my point of view, I am a co-owner of 10-people IT company, where we work 40 hour weeks with no overtime.