Sedation administration and agitation management are fundamental activities in intensive care. A lack of objective measures of agitation and sedation, as well as poor understanding of the underlying dynamics, contribute to inefficient outcomes and expensive healthcare. Recent models of agitation-sedation pharmacodynamics have enhanced understanding of the underlying dynamics and enable development of advanced protocols for semi-automated sedation administration. However, these initial models do not capture all observed dynamics. A physiologically-representative model that incorporates endogenous agitation reduction (EAR) dynamics is presented and validated using data from 37 critical care patients. Median Relative Average Normalised Density (RAND) values increase from 0.77 to 0.78 and minimum RAND values increase from 0.51 to 0.55 by adding EAR dynamics. These results show that both models are valid representations of the fundamental agitation-sedation dynamics. While the improvement is relatively small and the sensitivity of the model to the EAR dynamic is low, the inclusion of EAR is shown to be important for accurately capturing periods of low, or no, sedative infusion, such as during weaning prior to extubation.