We propose a model that combines simulation optimization and linear programming in order to define the shift pattern that best match the arrival pattern of patients in an Emergency Department (ED). The simulation model supplies the linear programming with the staffing levels that secure the performance of the ED, expressed in terms of the average Length of Stay (LOS) of patients. The linear model determines the shift scheduling of all employees with the use of the minimum cost, including several practical constraints as experienced in practice. The model includes also a heuristic that helps ED managers to find a solution that satisfies budget restrictions. This heuristic proposes an efficient manner of reducing the staffing cost, by decreasing the staffing levels in a way that affects the performance as less as possible. The application of the developed method leads to a reduction of 8.9% in the ED average LOS with the use of the same staffing budget.