A sofa bed is configured to be switched between a sofa configuration and a bed configuration. The sofa bed includes a base frame, a middle frame rotatably coupled to the base frame, a seat-and-foot frame rotatably coupled to the middle frame, a back-and-head frame rotatably coupled to the base frame, and a spring coupled between at least two of the base frame, the middle frame, the seat-and-back frame, and the back-and-head frame.