Why a standard sheet never fits a sleeper
A sofa bed mattress is about six inches thick and noticeably shorter than a regular bed. A twin flat sheet is cut for a nine to twelve inch mattress, so it either bunches under the sleeper or pulls off the corners overnight. The excess fabric then catches in the frame when the bed folds away, which is how sleeper mechanisms get damaged and how guests end up calling the front desk at midnight.
These are cut to the actual dimensions: Twin 38 x 72 x 6, Full 53 x 72 x 6, Queen 59 x 72 x 6.