The chemical lifetime of the mesospheric sodium layer is calculated from the eigenvalues and eigenvectors of the sodium chemical system. This method determines a lifetime that is an excellent estimate of the relaxation time of the sodium chemical system. The lifetime is more than a day in the vicinity of the mesospheric sodium layer and is much longer than the traditionally defined chemical lifetime for sodium of a few minutes. This verifies the assumption often made that transport determines the sodium perturbations in the presence of gravity waves and other rapid air motions. At the bottom of the sodium layer, below about 85 km, photochemistry is the dominant process.