The application discloses a mattress assembly which comprises a main mattress (1) and a supplement mattress attached thereto. The supplement mattress (1) comprises a main body having two lateral extensions (2), a footboard extension (3), a headboard extension (17), and a tubular structure (5). Extensions can be rotated 90 degrees or 180 degrees together with the tubular structure (5). A fitted sheet (20) is also provided. The surface of the mattress assembly can be expanded and it provides a safety means for preventing the user from falling to the ground.