Full-time work hours in continental Europe have recently both declined and become more flexible. However, evidence from France suggests that the reduction of work time has not clearly improved workers' well-being. This paper offers an explanation for this outcome using a simple bargaining model that analyzes the connections among work time, hours flexibility, and labor effort. The model suggests that in return for higher hourly wages, trade unions consent to greater management-controlled hours flexibility. Hours flexibility, in turn, leads to a deterioration in working conditions, including an intensification of labor effort, which becomes acceptable to workers only when work time is reduced. In this model, shorter work time does not reduce the overall effort of workers, and may indeed raise it. Consequently, workers' utility does not necessarily rise with reduced work time even if overall pay is unaltered.