In high performance sport, the goal is performance, not counting gruelling hours spent at the gym. The same approach could be translated into the workplace, where again the goal should be performance, not hours or days worked. This approach fits the increasing number of examples of companies raising productivity by moving to four-day weeks or working days finishing at 3pm.
These experiments need to move beyond detailing what the optimum number of hours should be. The point here isn’t to determine the best number of work days per week, the point is to determine the performance you are trying to achieve and then do what is required to deliver that.