This is a question to which I genuinely don’t know the answer. Is some component of sky glow caused by long-lived excitation and relaxation (aka phosphorescence) of molecular species in the upper atmosphere? Some so-called forbidden transitions have a lifetime of the excited state measured in minutes or hours. A commonly encountered example is found in alarm clocks where the hands and figures can remain glowing for hours after the lights have been switched off.
This mechanism may be a factor in the sky become darker long after it has been directly illuminated as the phosphorescence fades.