A coil-in-coil spring is provided that exhibits a variable loading response as the spring is compressed. The coil-in-coil spring comprises a continuous wire forming’ an inner coil having a substantially conical shape and an outer coil extending around the inner coil and having a substantially cylindrical shape. The inner coil includes a plurality of helical convolutions with diameters that progressively decrease as the plurality of helical convolutions extend from a lower end of the coil-in-coil spring to an upper end convolution of the inner coil. The inner coil also has an uncompressed height that is about 75% of the uncompressed height of the outer coil, A mattress is further provided and includes the coil-in-coil springs arranged in a matrix.