Because when her husband comes to bed she must conform to "sanity" and her role of wife, she is behind bars, in prison.
Explanation:
the speaker described where she resides as A colonial mansion and a haunted house. She said "I shall have to get back behind the pattern when it comes night, and that it hard!" simply because once her husband comes over to bed, the speaker must exercise sanity as well as role of a wife which made her felt like she's staying behind the bar as a prisoner
