Re sets in the western mountain
And lights the netherworld with his rays.
This means: Re rests in Osiris,
Osiris rests in Re, this means.
We must thus recognize this mummy as the form of the dead sun god, who does not become Osiris, like the other dead, but who “rests in Osiris,” that is, who unites with Osiris only temporarily and then once again separates himself from him. This form, which comprises the two gods, is called the “United Ba”:
United Ba Re-Osiris,
Great God who is at the head of the West in Abydos.
Thus the story of the birth of the children where the gods arrive at the house of Reusre to help Ruddjedet likely means "below the horizon, following the sun set".
David Lorton translation of Jan Assmann, Death and Salvation in Ancient Egypt Translated from the German by David Lorton), Cornell University Press, 2005, p188, p192