There are three different sleeping positions and each of these can lead to different choices in a mattress. Of course almost everyone sleeps in different positions through the night but most of us have our favorite or at least our favorite two. Lets take a look at each of them and how they can affect which mattress may work best for you.
Side sleeping is the most common sleeping position and because it has a more "curvy" profile than the other two positions, pressure relief becomes an important part of the choice here so you don't end up sleeping on your more pointy or bony parts or to use a technical term ... your bony prominences. These are of course your hips and your shoulders. There are deeper gaps to "fill in" to spread your weight out on the mattress when you are sleeping on your side so side sleepers need a deeper cradle to relieve pressure so the gaps are filled in and help spread the weight over the mattress. In other words they need to "sink IN" to the comfort layers more. This means that the comfort layers generally need to be a little softer and/or thicker than the other 2 positions. In general a good starting point for a good comfort layer for a side sleeper is 3" and then depending on weight, body shape, preferences, and the firmness of the support layers, to go up or down from there. Most side sleepers will fall in the range of from 2" -4". Without an appropriate comfort layer, a side sleeper will end up with pressure points during the course of the night and could have symptoms of numbness, soreness, localized redness, or end up tossing and turning all night as your body tries to relieve the pressure.
Back sleeping is the second most common position and has a slightly less curvy profile than side sleeping. The "gap" that needs to be filled in with the comfort layer for pressure relief is generally the small of the back (the lumbar area) and because the "gaps" in your profile are not quite as deep as in side sleeping, a slightly thinner top layer will generally work a little better. In back sleeping in other words a slightly shallower cradle is needed. A good starting point for a back sleeper is 2" of softer material on top of your mattress and then increasing or decreasing from there depending on other factors like weight, preferences, or other sleeping positions.
Stomach sleeping is the least common position and has the flattest sleeping profile of all the positions. Stomach sleepers have fewer gaps to fill in and need a thinner, firmer top comfort layer than the other positions. It is especially important for a stomach sleeper to avoid hyperextension of the lumbar area. A good place to start is 1" of softer material so there is enough "softness" to cushion the bony parts of the pelvis. In general, stomach sleepers should choose the thinnest firmest comfort layers that are comfortable as sinking in too far can lead to a swayback position and cause back issues. Stomach sleeping is the most prone of all sleeping positions to sinking down too far in the pelvic area.
Combination sleeping is the most common of all and most people fall into this group although different people spend more or less time than others in different sleeping positions. Combination sleeping is also more difficult to deal with than someone who sleeps in one position since the different positions have different basic requirements. Since most of us fall in this group, it is important to choose a mattress that is able to keep you comfortable in all your sleeping positions. Fortunately better quality materials and constructions have a range of different positions they can respond to and even extreme differences in profile (such as stomach/side sleepers) can be accommodated with different materials and styles of mattress. With combination sleeping it is usually wise to choose a comfort layer that is a little thinner than your "deepest” sleeping position would normally require (typically side sleeping) and then choose a support layer underneath that helps you to sink in a little extra when you need it. A middle layer or "transition layer" can be especially useful for those who sleep in multiple positions and it can help you to sink in enough to help with pressure relief and also help keep you from sinking down too far and causing back issues. Innerspring designs that are more conforming like pocket coils or offset coils or different foams that are softer on top and become firmer faster when they are compressed (such as latex) also make very good choices here.
The support layers underneath the upper softer comfort layers are of course important for all 3 positions but different sleeping positions will need different combinations here. If you are choosing a thinner comfort layer which needs a little bit of help in forming a cradle (especially for combination sleeping), or if you change positions during the night (which most people do), then a more conforming support layer is appropriate which means a little softer. Conforming innersprings like pocket coils and offset coils or middle layers of foam that are "in between" the softer comfort layer and the firmer bottom part of the support layer make good choices. If you choose a slightly thicker comfort layer which already forms a perfect cradle for your sleeping position and you don't change positions a lot during the night, then a firmer support layer underneath is important to stop any further sinking down of the heavier parts of your body. In these cases a middle layer is often not even needed and the support layer may be a single thicker layer instead (just an innerspring or a single thicker layer of firmer foam). Higher quality materials that become increasingly firm at a faster rate with deeper compression can also eliminate the need for a middle or transition layer.
So to recap, the different sleeping positions are part of what determines how deep a cradle you need and how thick the comfort layer should be. This in turn affects the type of support layers that will be appropriate. The layers underneath this comfort layer can help with pressure relief for those who change positions and with thinner upper layers or can be primarily focused on preventing you from preventing your heavier parts from sinking down too far with thicker comfort layers.
Of course, your weight and body profile will also make a difference in how far you tend to sink in and how deep a cradle you need in your mattress so for more on your "statistics, and how they can affect your choices ... read on.