An adjustable bed includes a plank assembly, a support assembly, a frame and a power unit. The plank assembly includes a first waist plank and a second waist plank. The power unit is pivotally connected to the frame. The support assembly includes a waist-lifting unit that includes a first lifting element pivotally connected to a portion of the frame, a second lifting element pivotally connected to another portion of the frame, and a transmission unit operatively connected to the first and second lifting elements.
