A coil-in-coil spring is provided that exhibits a variable and non-linear loading response upon compression. The spring comprises a continuous wire including a first spring portion having a first spring constant, a second spring portion having a second spring constant less than the first spring constant, and a third spring portion having a third spring constant less than the first spring constant and less than the second spring constant. In the coil-in-coil spring, the first spring portion forms an outer coil of the coil-in-coil spring and the second spring portion, the third spring portion, or both the second spring portion and the third spring portion are positioned within the first spring portion and form an inner coil of the coil-in-coil spring. A multi-coil spring is also provided that includes an outer coil, an intermediate coil positioned within the outer coil, and an inner coil positioned with the intermediate coil.