Mattress arrangement, such as a bed, having adjustable firmness

A mattress arrangement, such as a bed arrangement (1), having adjustable firmness is disclosed. The mattress arrangement comprises an upper mattress (3) and a lower mattress (4) being arranged beneath said upper mattress (3). At least one side of the lower mattress (4) is moveable in relation to an opposite side of the lower mattress, the lower mattress (4) thereby being expandable into an expanded state, having a lower firmness, and compressible into a contracted state, having a higher firmness. A support layer (6) is arranged between the upper mattress (3) and the lower mattress (4), to partly support the upper mattress (3), and arranged overlying the at least one moveable side of the lower mattress (4). Hereby, at least one compartment is formed beneath the upper mattress (3), wherein the compartment(s) is at least partly empty when the lower mattress (4) is in the contracted state and at least partly filled when the lower mattress (4) is in the expanded state.