We present a steady state model of the midlatitude nighttime sodium layer which is comprehensive in the sense that it includes deposition of the metal, diffusion, and kinetics as well as the seasonal variations in the atmosphere. The model is used to compute seasonal variations in the layer parameters and the sodium nightglow. The behavior of the model reproduces well the observed behavior of the layer. We find that the main contributor to the seasonal variation is the temperature dependence of the reaction recycling atomic sodium from NaHCO3. Seasonal variations in the atmospheric minor species including ozone are secondary effects at best. The relative contributions of all factors are compared. ¿ American Geophysical Union 1995