Designing the working day is not just a matter of changing the shift system. It also requires reflection on the working conditions and a negotiation of solutions with the stakeholders concerned. This article seeks to show that a participatory approach, built using organisational simulation, provides a framework with which to understand the reality of each profession and co-construct suitable solutions. Our action-research took place in a hospital's pneumology ward. The methodology can be broken down into four phases: diagnosis, sharing of the diagnosis, organisational simulation (the focus of this article) and experimenting with solutions. The results show that the approach gave the stakeholders the opportunity to discover and discuss the rules and constraints of actual work, to compare their different views, and to develop a new and shared view of the work situation. The approach allowed them to co-construct relevant solutions and to appropriate the changes necessary for their success.