In addition to the previous response,
The desperation of the woman wanting to free the woman who she feels is trapped inside the yellow wallpaper affects the move to the story in that it causes the reader to question the mental stability of the protagonist. This also causes the story to take a detour from sadness and nostalgia to mental angst, and finally to the liberation of all fears and emotions through the breakdown of the woman. This provides the story with a complete tour de force of emotions that range from the sublime to the extreme, and gives the reader a range of different feelings that will end with the eventual "piece de resistance" where both the reader and the protagonist will want the pain to end in some shape or form.
Charlotte Gilman Perkins's "The Yellow Wallpaper" is a story in which the wallpaper has become symbolic of the cult of the Victorian Womanhood. The wallpaper of a "repellent" yellow--which represents decadence--appears different in the daytime from at night to the unnamed narrator. She remarks, "This paper looks to me as if it knew what a vicious influence it had!"
Perkins's narrator observes that there is a kind of sub-pattern in a different shade that can only be seen in certain lights:
But in the places where it isn't faded and where the sun is just so--I can see a strange, provoking, formless sort of figure, that seems to skulk about behind that silly and conspicuous front design....On a pattern like this, by daylight, there is a lack of sequence, a defiance of law, that is a constant irritant to a normal mind....the pattern is torturing....
The narrator remarks, also, that the pattern changes as the light changes. At night with the moonlight, the pattern becomes "bars" and the woman behind it is as plain as can be. By daylight she is "subdued." The narrator feels that the daylight subdues the woman. It seems apparent that the changes in the yellow wallpaper reflect what transpires with the narrator herself. Thus, there is a disturbing mood, a tone of foreboding rebellion, as she becomes obsessed with the wallpaper, finally feeling that she must free the woman behind it.
During the day society is watching. At night, the narrator is free to pursue any frowned upon activities that are oppressed under the watchful eyes of society. Jennie and John are absent from her nighttime adventures. The women behind the bars come to represent the struggles women endure that are present in Victorian culture. It is only under the guise of night, away from society's prying eyes, that women can express their anxieties. The anxieties about their inability to engage in intellectual and meaningful activities. For if they indulged these fancies, they were considered a heretic. Women were expected to aspire to nothing more than domestic duties, as the narrator points out in her sister-in-law Jenny. What is interesting in the narrator's assessment of Jenny is that she seems to pity her lack of aspirations. For all the trauma the narrator endures, it is she who pities Jenny.