A mattress has two basic roles — that is, to support and to provide comfort.
What is support?
Support is specifically about spinal alignment, that is, the ability of a mattress to keep your spine aligned while you are sleeping just as when you are standing up straight with good posture. The spine is straight from top to bottom (when viewed head on), and when lying on your side, should remain as such. However, when viewing the spine from the side, the “S” curve is revealed, where the small of the back and the neck bends inward and the
upper back and pelvis bends outwards. Therefore, a person sleeping on their back or stomach needs a mattress that keeps the spine in the natural curved position. The heavier part of your body needs to be supported, and held in place, while the curved parts take advantage of “push back”. This cavity is filled with the secondary center support layer of the mattress. The middle layers of a mattress play a dual role and can help to differing degrees with both sinking in (for pressure relief and comfort) and sinking down (for spinal alignment and support) depending on the construction of the mattress.
Basically, the supportive core of a mattress needs to control how far your body, as a whole, and also different parts, sinks into the mattress.
The myth that firmer mattresses offer better support are long gone as more people recognize that spinal alignment needs a mixture of softness and firmness and is interconnected with how well a mattress relieves pressure. There are two parts to the supportive core of a mattress – the main push back from the coils and then the secondary support, a transition layer, which is a function of the upper cradle as it allows you to fill in gaps by using better materials in the comfort layers that can also push back on the lumbar, which is the biggest gap in the human profile.
Since very few people sleep in the same position all night long and that their support needs change constantly through the night as they change positions, a mattress that has a larger range of positions is required. Thus, your mattress choice should not only have good push-back, but a good center support that relieves pressure.
What is comfort?
Comfort is about pressure relief, where your mattress conforms to your body shape and forms a cradle that is deep enough to disperse your weight over the surface of the mattress and relieve pressure points. The top part of the mattress forms the comfort layer. Thinner comfort layers often need the help of the layer below it, a transition layer. These middle layers of a mattress play a dual role and can help to differing degrees with both sinking in (for pressure relief and comfort) and sinking down (for spinal alignment and support) depending on the construction of the mattress. The main reason for the different materials used in the comfort layers of a mattress is to spread out the pressure of your weight in many different sleeping positions. This is similar to the feeling of floating on water.
All mattresses are made up of different layers with different degrees of both softness and firmness in the different layers. That softness and firmness is very much based on individual perception and is hard to measure, though the ILD (Indentation Load Deflection) and IFD (Indentation Force Deflection) measures are in place, which indicates how much pressure is exerted to indent the foam a certain amount
Generally, when talking about mattress comfort, you should be referring to the upper comfort layers, which offer pressure relief, and is typically the softness of a mattress. This is the “feel good” part of the mattress. When talking about support, you should be referring to the spinal alignment and support, and is typically the firmness of a mattress. This is the “good for me” part of the mattress.