Night and day play huge roles in Mariana's life—and pretty similar ones, too.
In the second stanza, Mariana cries while the morning dew sits on the grass: "Her tears fell ere the dews were dried" (14). She's still crying when the sun comes out and dries it.
Normally, daylight and dew on the grass are positive associations, but Mariana is so sad that she can only feel her sorrow. Stanza three sees her wake at night and stay until daytime: "Without hope of change,/ In sleep she seem'd to walk forlorn" (29-30). Talk about dreary.
And then, the poem returns to night, and day again, and on and on. This cycle only serves as a symbol that emphasizes her loneliness and growing despair.
Neither night nor day offer any comfort to Mariana—bummer.