According to observational data, the nighttime variation of the integral emission of the upper atmosphere in the red line (630 nm) at midlatitudes has a number of characteristic properties such as the emission growth during midnight and twilight periods. The appearance of these peculiarities depends on season and on the level of solar and geomagnetic activity. To study mechanisms of generation of the observed variations in the red emission the calculations of the variation of these values have been carried out based on the model of the ionosphere-plasmosphere interaction under different geophysical conditions (winter, spring, summer, fall, F10,7 = 70, 130, 200). This model takes into account the nitric-oxygen cycle of chemical reactions, which is basic at altitudes higher than 100 km, and includes the ion composition (O+, N+, H+, He+, O2+, NO+, N2+), temperatures of electrons and ions, photoelectron fluxes and horizontal neutral wind. Analysis of calculated results has made it possible to assess quantitatively the contribution of variations of the neutral composition, wind velocity, and conditions of the emission to the formation of the observed night emissions of atomic oxygen at the 630-nm line.