This paper reports on an extensive analysis of the degradation of AlGaN/GaN HEMTs submitted to on-state stress tests. By means of combined electrical and electroluminescence characterization we demonstrate that: (i) exposure to on-state stress can induce a remarkable decrease in drain current; (ii) degradation rate strongly depends on the intensity of the EL signal emitted by the devices during stress, while it has a negligible dependence on temperature. On the basis of the experimental evidence collected within this work, degradation is ascribed to electron trapping in the gate-drain access region, induced by hot electrons. Finally, we derived an acceleration law for GaN HEMT degradation, by using the intensity of the EL signal as a measure of the stress acceleration factor.