In The Awakening, it appears that Edna's transformation begins to take place at night, specifically when she finds that she can swim. She has a moment of elation at the freedom she feels, and her moment of enlightenment tells her that she is her own person and that no one owns her; that she need not sacrifice who she is for anyone.
The imagery of darkness, night and shadows supports the sense of the mysterious, what is hidden from view, and a time when secretive things take place; it also seems to support her growth as a free-thinking woman, a new creation, that much of what she does when she is exploring these new sensations occurs at night: in the hammock after swimming, at her dinner party, with Arobin, returning to Robert after sitting with Adele, etc.
As Edna has been living until now in a world of shadows, being someone's daughter, wife and mother, she has not fully realized who she is and what she has the right to expect from life.
When Edna begins to change, only Mademoiselle Reisz knows why, as Edna tells no one else about Robert. Certainly it is not necessary that she has changed because of Robert, but that once she is "awakened," she sees the world, herself, and life's possibilities that were until now hidden from her sight, including some kind of life with Robert--except that not even Robert can see this. She is enlightened; he is not.
Even so, it is not until the story's end at the beach, while she is standing at the water's edge, naked and fully exposed to the sunshine that everything becomes clear to her. Feeling both "strange and awful," as well as "delicious," she faces the light, leaving the shadows and darkness behind and chooses to face her fate on her own terms because of the woman she has become.