This article gives an account of research carried out on working hours in a French hospital and specifically the impact on two ways of organizing working hours (12 hour shifts vs 8 hour shifts) on workers health and job strain as perceived by the health care providers. Based on 255 questionnaires, this research indicates that the health care providers who work in 12 hour shifts and 8 hour shifts have the same perception of their health and job strain. However, there are some differences: the health care providers working in 12 hour shifts feel less fatigue and emotional exhaustion than the health care providers working in 8 hour shifts. We discuss these results taking into account that: i) breaks in 12 hour shifts are longer than in 8 hour shifts; ii) workers have better relationships with their colleagues in 12 hour shifts, based on consultation and a vote held when a hospital department thought about changing to 12 hour shifts and iii) the workers volunteer to work in a 12 hour shifts.