Spring with variable resistance and mattresses including the same

A spring, comprising:

• a plurality of spring elements that are stacked atop one another, each spring element comprising an upper member and a lower member connected to the upper member, each spring element defining an internal space between the upper member and the lower member, and each spring element configured to compress when a force is applied to the spring, such that the upper member moves toward the lower member;
• a first spring stop positioned in the internal space defined by a first spring element and preventing the first spring element from compressing past a first predetermined compression distance;
• and
• a second spring stop positioned in the internal space defined by a second spring element and preventing the second spring element from compressing past a second predetermined compression distance.

A mattress, comprising:

• multiple springs arranged in a matrix and defining a first support surface and a second support surface opposite the first support surface, with each of the multiple springs including
• (a) a plurality of spring elements that are stacked atop one another, each spring element comprising an upper member and a lower member connected to the upper member, each spring element defining an internal space between the upper member and the lower member, and each spring element configured to compress when a force is applied to the spring, such that the upper member moves toward the lower member, and
• (b) one or more spring stops positioned in the internal space defined by on one or more of the plurality of spring elements;
• an upper body supporting layer positioned adjacent to the first support surface;
• a lower foundation layer positioned adjacent to the second support surface;
• and
• a sidewall extending between the upper body supporting layer and the lower foundation layer.