We have studied the electroluminescence and photoluminescence of Nichia single-quantum-well Al0.2Ga0.8N/In0.45Ga0.55N/GaN green light-emitting diodes over a broad range of temperatures (15-300 K) and currents (0.2 μA - 2 A). The most striking behavior is an anomalous temperature shift of both photo- and electroluminescence, with the emission peak moving towards higher energies with increasing temperature. This blue shift is opposite to that of the energy gap of the active layer, which practically excludes interband transitions as responsible for the observed optical transitions. We suggest that population effects within the band tails can account for the observed anomaly. We also determined that the current flowing through the p-n junction is dominated by carrier tunneling, the omnipresent effect in the GaN-based optoelectronic devices.