Please analyze the quote below from "The Yellow Wallpaper."
At night in any kind of light, in twilight, candle light, lamplight, and worst of all by moonlight, it becomes bars! The outside pattern I mean, and the woman behind it is as plain as can be.
I didn't realize for a long time what the thing was that showed behind, that dim sub-pattern, but l now I am quite sure it is a woman.
By daylight she is subdued, quiet. I fancy it is the pattern that keeps her so still.
This is a very important quote from the novel because it explores how the narrator, in her state of sliding gently into ever-greater madness, comes to identify her own state and position as being an intellectually frustrated woman forced to stay in bed and have "rest" for her own good with the position of the woman that she "sees" behind the pattern of the yellow wallpaper, that she equates with being like "bars" on a prison cell. Let us remember that the point of view of this story is first person, and thus we can see how the narrative is told from the perspective of the woman herself, and thus is unreliable. What she is seeing is not actually a woman trapped behind bars, but she is projecting herself into this imagined figure behind the "bars" of the wallpaper to express her own sense of frustration and impotence as she, like the woman she has created, is trapped and restricted by the bars of patriarchal society.
Let us note the similarities between these two figures. Both are "quiet, subdued" characters during the day, as the narrator is forced to play the role that she feels is expected of her by her husband, who, in caring for her, unwittingly spurs on her madness. However, it is at night when both figures are free to express their rage and frustration by shaking against the bars that imprison them so effectively when there is nobody to see them. Thus the importance of this quote lies in the way it psychologically reveals the sense of impotence and powerlessness experienced by the narrator.
There are two possible interpretations to this quote. As you know, "The Yellow Wallpaper" is about a woman's slow descent into madness, which is actually based on Charlotte Perkins Gilman's own dealings with mental illness (she suffered from something called "nuerasthenia").
From a psychological perspective, the prescription for "rest" is enough to drive anyone insane. The unnamed narrator is deprived of any possible stimulation and so, invents a persona dervied from the "woman" she finds hiding in the wallpaper, especially after the thing that aids in her cure--writing--is deemed too expressive and is eventually forbidden. If you think of it from the restrictions placed on 19th century women and their marginalization, you can certainly understand how she had to find a 'companion', even if it was an imaginary character stuck behind "bars" in the wallpaper--no small indication of her own life.
From a feminist perspective, this is Gilman's attempt to show how restrictive society and marriage were for women. She makes no decisions in the story and defers all opinions to either her husband, the nanny or her 'keeper.' Here, the focus is on domestic victimization. Thought the character may have had a legitimate mental illness, it's itensified by the domestic structure she resides in. She is literally rendered voiceless and powerless by patriarchal society.
And so your quote, much like the rest of the story, is a 19th century woman's gentle diatribe at how she, too, must remain "subdued, quiet."