Encased hourglass coils and encased hourglass coil mattress cores have steel wire coils with an hourglass profile encapsulated in a sheet material enclosure, package, casing, housing, containment or encapsulation, wherein coils are enclosed within an enclosure made of fabric, non-woven, or other material which encapsulates each individual coil spring and serves to maintain multiple coil springs in an array or alignment in a mattress core. The hourglass configurations of the coils, wherein at least one helical convolution of the coil body located between the coil ends has a diameter which is less than every other convolution of the coil body, provides a mattress core which has a relatively lower spring rate upon initial compression and a higher spring rate at higher compression loads.