Discrete-event simulation (DES) has been widely used in systems analysis. However, authors defend the idea that this kind of simulation does not properly represent the human element or the factors affecting its productivity. In Agent-Based Simulation (ABS), agents are intelligent, autonomous and proactive beings, which are features closely related to human behaviour. Therefore, ABS and DES were combined into one model to represent the human element as agents, who were subjected to one of the factors that affect their performance, the circadian rhythm. After statistical validation it was determined that the computer models integrating ABS and DES represent the productive systems under analysis with high level of confidence. A comparison between the model built and a DES model where operators are represented by resources was also performed. It was verified that the ABS/DES represents better the actual systems behaviour than the DES, showing a lower average of relative error.