Computations of Schumann resonance (SR) field amplitudes from a point source in the uniform and nonuniform cavities are performed to estimate the relative importance of the day-night asymmetry in observed SR amplitudes. Additional simulations with globally distributed sources, representing diurnal and seasonal variations of global lightning activity, are performed in uniform and nonuniform cavities to evaluate the impact of the day-night asymmetry on the diurnal and seasonal amplitude variations. The results show that the effect of diurnal ionosphere changes on the first mode SR amplitudes is ~10%. Model results indicate that the source properties (the level of activity and proximity to the observer) play the dominant role in the diurnal variations observed in SR field records, with the day-night asymmetry being secondary in importance.