From the article: "companies around the world that have cut their work week have found that it leads to higher productivity". I'm disappointed that the article

From the article: "companies around the world that have cut their work week have found that it leads to higher productivity". I'm disappointed that the article does not even explore the question, at what point does productivity decrease? It says that going from 5 to 4 days brought about an increase. The natural question is, what would happen if you went to 3? To 2? 1? Certainly at 0 days per week, productivity will decrease. And I know it will be a long time before even a 4-day week is the norm. But at least as an academic exercise, why is no one asking the question? What is the fewest number of hours per week that yields the same amount of productivity as 40? (Such a study would have use averages, of course.)