The energy transitions of GaAsSbN/GaAs strained-layer single quantum wells (QWs), grown by molecular-beam epitaxy, are studied in detail, using photoluminescence (PL) and photoreflectance (PR) spectroscopies. The optical transitions energy observed in the PL and PR spectra of GaAsSbN/GaAs QWs show a strong decrease with a small increase in the N composition. These effects are explained through the interaction between the conduction band and a narrow resonant band formed by nitrogen states in the GaAsSbN alloy. The temperature dependence of ground-state energy of strained-layer QWs is analyzed using the Bose-Einstein relation in the temperature range from 9 to 295 K. The parameters that describe the temperature variations of the ground-state energies are evaluated and discussed.