PrincessanthPea,
This is one of those questions that can be much more complex than it seems at first.
First of all … “softness” has different meanings to different people and there are different “species” of softness (for lack of a better description). Post #4 here talks a little more about this. different people will be more or less sensitive to either the “feel” of softness, the pressure relief softness, the support softness, or the “overall” softness of a mattress and each of these may be talking about very different things when they talk about the softness of a mattress.
Softness is also relative to the body type and sensitivity of each person. People who are heavier that sink into a foam will feel it is softer than someone who is lighter and the same foam doesn’t allow them to sink in as much. Some people will also be more sensitive to some of the deeper layers or they may come into play more with higher weights and some will tend to focus on the “feel” of the top layer or even the “feel” of the cover.
Because of all the varying descriptions of what soft and firm really is … it’s usually much more “accurate” and objective to talk in terms of the overall “feel” of a mattress (surface feel), the pressure relief of a mattress, and the support and alignment of a mattress.
The most common definition of softness has to do with the pressure relieving qualities of a mattress. This comes from the upper layers of the mattress and how well they re-distribute weight away from any pressure points. If you read post #6 here it describes someone floating in the air in good alignment and gradually being lowered onto a mattress. At first only the more “pointy parts” of the body will contact the mattress and all the weight is concentrated there. As you gradually sink in deeper more of the body surface begins to take up weight and relieve pressure on the parts that were initially in contact with the mattress. Eventually enough of the body surface is bearing weight that there are no perceptible pressure points. This is why the depth of the pressure relieving cradle and the materials that are used in the top layers are so important.
Generally … if pressure on the surface capillaries in the pressure points are reduced to below about 32 mmhg (a unit of pressure) for most people, then the capillaries won’t collapse with the pressure and the blood flow will be maintained. When this is achieved … the body doesn’t feel “pressure” or the urge to move and restore blood flow. Once you have reduced the pressure on the pressure points to the maximum possible … then there is no further benefit to sinking in any more. If the heavier parts keep on sinking because the comfort layers are softer and thicker than you need to achieve maximum pressure relief … then all that happens is that you lose alignment with no more benefit of more pressure relief.
Some people need more pressure relief than others because they are either more sensitive, have less muscle tone, sleep in more pressure prone positions, or because their capillaries “collapse” more easily for various reasons. These people will generally do best with the maximum possible depth of cradle that is beneficial but more than this introduces new risks for alignment that have no pressure relieving benefit.
In come cases … some people just prefer to sink in more deeply into the comfort layers and as long as this is done evenly without one part sinking in too much relative to the rest … then this is a matter of preference.
Sometimes some people aren’t as sensitive to sleeping in a position where spinal or joint misalignment causes issues (they may be more flexible for example) so for these people it’s easier to just add more softness for extra pressure relief with less thought of the alignment issues that can go with it. For these people … a little too much softness and thickness in the comfort layers isn’t as likely to cause back pain or joint pain that can come from a misaligned spine or joints.
In other cases … people just feel better with a nice fluffy layer on top of the mattress in addition to comfort layers that improve pressure relief just because it “feels” soft even though it doesn’t necessarily improve pressure relief and in some cases can even reduce it.
Finally there are slow response materials such as memory foam which change in softness depending on temperature, humidity, and the length of time they are compressed and can also feel firmer when you move or change position than they do when they have warmed up underneath you. They respond more slowly which can affect how soft or firm they feel to different people and with different sleeping styles and they can change their firmness level depending on the type and thickness of the layers over them (including any mattress pads, toppers, or sheets and bedding) which can change the amount of heat which reaches them and their softness and response (see post #9 here and post #8 here for more about the differences between different types of memory foam).
So the answer to your question is that different people need a different depth of cradle to achieve what for them is “enough” pressure relief. Underneath this they need firm enough (for them) layers to “stop” further sinking down of the heavier parts of the body to keep the spin in good alignment. In addition to this some may also add a nice plush layer on top just because it feels better to them. “Enough” to relieve pressure for that particular person without causing alignment issues is always the key. For some people … “enough” by any definition of softness is much different than others.
Phoenix