The line profiles of the sodium D-lines in the night airglow show considerable broadening which can not be ascribed to temperature alone. Based on this broadening, the known chemistry, and physics of sodium atom production in the upper atmosphere at night, we deduce that excited sodium atoms are most likely produced from the reaction of sodium monoxide and atomic oxygen, and thus find an upper limit for the dissociation energy of sodium monoxide equal to 2.89 eV (determined to a precision of 0.02 eV).