how is it significant that the wallpapers design turns into bars in the moonlight?

This is when she felt most alone." in twilight, candlelight, lamplight, and worst of all by moonlight, it becomes bars!"

In "The Yellow Wallpaper," moonlight represents as time for the feminine. During the day, the narrator writes that the woman trapped in the wallpaper is motionless and immobile. As moonlight strikes the wall, however, the woman begins to move or, perhaps more accurately, to creep. This pattern mirrors the narrator’s own daily movements. During the day, she sleeps; at night she lies awake, alert, and invested in the intellectual activity that she must suppress during the day while her husband is watching.