An aircraft passenger seat (100, 600) configured to facilitate ergonomic alignment of a passenger while in a sleeping position, comprising: a seat bottom; and a seatback comprising: a central opening corresponding to a shoulder region of a passenger lying in a side sleeping position and configured as a recess provided in the seatback; and a side sleep support assembly mounted within the central opening, the side sleep support assembly including an adjustable support spanning a central portion of the seatback, wherein the adjustable support is configured for at least one of translation and deformation; wherein the seatback includes a seatback frame which is a generally planar, rigid member that provides structural support and dictates a vertical and lateral extents of the seatback, the seatback including a back portion surrounded by a frame edge configured to receive a cushion system, the cushion system being secured to the back portion at a number of securing locations; and/or wherein the seatback includes a seatback frame surrounding the central opening, the sleep support assembly being arranged within the central opening and being securable to the surrounding seatback frame, a back portion of the seatback frame including a securing edge (114a-b) on each lateral side of the central opening and including a number of securing locations (116a-b) along each lateral side of the securing edge (114a-b); Z r characterized in that the adjustable support comprises: a flexible diaphragm spanning a central portion of the side sleep support assembly; a plurality of fasteners applying tension to the flexible diaphragm in at least one of a latitudinal direction and a longitudinal direction of the seatback, and at least one actuator, each actuator of the at least one actuator configured to cause adjustment of one or more of the plurality of fasteners connected along opposing sides of the adjustable support for connecting the adjustable support to the seatback, wherein the adjustable support is generally rectangular in shape and the at least one actuator is configured for causing at least one of the translation and deformation of the adjustable support; the seat comprising a controller configured to control the one or more actuators responsive to seatback movement to a lie-flat sitting position to modify a density and/or shape of the side sleep support assembly. a seat bottom; and a seatback comprising: a central opening corresponding to a shoulder region of a passenger lying in a side sleeping position and configured as a recess provided in the seatback; and a side sleep support assembly mounted within the central opening, the side sleep support assembly including an adjustable support spanning a central portion of the seatback, wherein the adjustable support is configured for at least one of translation and deformation; wherein the seatback includes a seatback frame which is a generally planar, rigid member that provides structural support and dictates a vertical and lateral extents of the seatback, the seatback including a back portion surrounded by a frame edge configured to receive a cushion system, the cushion system being secured to the back portion at a number of securing locations; and/or wherein the seatback includes a seatback frame surrounding the central opening, the sleep support assembly being arranged within the central opening and being securable to the surrounding seatback frame, a back portion of the seatback frame including a securing edge (114a-b) on each lateral side of the central opening and including a number of securing locations (116a-b) along each lateral side of the securing edge (114a-b); Z r characterized in that the adjustable support comprises: a flexible diaphragm spanning a central portion of the side sleep support assembly; a plurality of fasteners applying tension to the flexible diaphragm in at least one of a latitudinal direction and a longitudinal direction of the seatback, and at least one actuator, each actuator of the at least one actuator configured to cause adjustment of one or more of the plurality of fasteners connected along opposing sides of the adjustable support for connecting the adjustable support to the seatback, wherein the adjustable support is generally rectangular in shape and the at least one actuator is configured for causing at least one of the translation and deformation of the adjustable support; the seat comprising a controller configured to control the one or more actuators responsive to seatback movement to a lie-flat sitting position to modify a density and/or shape of the side sleep support assembly.