It would be a tight squeeze, but could be done just for sleeping, then deflate the mattress and put away in the morning. We have survived with a pack n' play crib in the room, we put it by the window and it left just enough room to walk between it and the bed. I don't think a twin mattress will leave you any room between it and the bed, just be sure no one wants to get up from that side of the bed during the night!