The design consists of the features of shape and configuration of the MATTRESS as illustrated in solid lines in the attached drawings. Image Description: FIG. 1 is a top front perspective view of a first variant of the MATTRESS, which consists of three mattresses of the same thickness; FIG. 2 is a front elevational view of the mattress of FIG. 1 (rear elevational view is identical); FIG. 3 is a left side elevational view of the mattress of FIG 1. (right side elevational view is identical); FIG. 4 is a top plan view of the mattress of FIG. 1; and FIG. 5 is a bottom plan view of the mattress of FIG. 1; FIG. 6 is a top front perspective view of a second variant of the MATTRESS, which consists of two thin mattresses on a thick mattress; FIG. 7 is a front elevational view of the mattress of FIG. 6 (rear elevational view is identical); FIG. 8 is a left side elevational view of the mattress of FIG 6. (right side elevational view is identical); FIG. 9 is a top plan view of the mattress of FIG. 6; and FIG. 10 is a bottom plan view of the mattress of FIG. 6; FIG. 11 is a top front perspective view of a third variant of the MATTRESS, which consists of the configuration of two thin mattresses under a thick mattress; FIG. 12 is a front elevational view of the mattress of FIG. 11 (rear elevational view is identical); FIG. 13 is a left side elevational view of the mattress of FIG 11. (right side elevational view is identical); FIG. 14 is a top plan view of the mattress of FIG. 11; and FIG. 15 is a bottom plan view of the mattress of FIG. 11; FIG. 16 is a top front perspective view of a fourth variant of the MATTRESS, which consists of the configuration of a thick mattress between two thin mattresses; FIG. 17 is a front elevational view of the mattress of FIG. 16 (rear elevational view is identical); FIG. 18 is a left side elevational view of the mattress of FIG 16. (right side elevational view is identical); FIG. 19 is a top plan view of the mattress of FIG. 16; and FIG. 20 is a bottom plan view of the mattress of FIG. 16; FIG. 21 is a top front perspective view of a fifth variant of the MATTRESS, which consists of the configuration of a thin mattress on two mattresses of the same thickness; FIG. 22 is a front elevational view of the mattress of FIG. 21 (rear elevational view is identical); FIG. 23 is a left side elevational view of the mattress of FIG 21. (right side elevational view is identical); FIG. 24 is a top plan view of the mattress of FIG. 21; and FIG. 25 is a bottom plan view of the mattress of FIG. 21.