Hi sretodd,
It sounds as if the alignment of this mattress agrees with you more than your previous choices, so that’s a good thing. While I can’t diagnose why your arms are feeling numb, I can provide some insight and possible things to investigate.
Technically, limbs “going numb” are usually caused by a lack of pressure point relief.
Each person has a range of tolerance for both pressure relief and alignment (the two main functions of a mattress) and there are many methods that are part of mattress theory and design to deal with both of these and how they interact so that both are within the range of what someone needs and prefers in their mattress. While pressure is never relieved down to “zero” … as long as it is within the tolerance of the individual both in terms of their perception and comfort and in terms of any restriction of the capillaries and blood flow in the areas of the “bony prominences” then different types of mattress construction and materials can achieve both the pressure relief and alignment that someone needs yes. There’s lots more information about this in the “mattresses” section of the site and in the more detailed information at the end of this post that describes how and why different materials and types of construction can be used to achieve both.
There is little consensus information about pressure relief … even in a hospital setting that deals with treatment of pressure ulcers and there is a great deal of information that needs further research. Technically, to qualify as a “pressure reducing support surface”, a mattress needs to reduce pressure to below 32 mmHg (also called a Torr) on “most bony prominences most of the time”. Bony prominences are the bones and joints in each sleeping position that protrude and have less tissue between them and the surface. To qualify as a “pressure relieving support surface” pressure needs to be reduced to below 32 mmHg (Torr) on “all bony prominences all of the time”. This is also not quite accurate as fixed numbers like this are not considered to be completely accurate or valid anymore as research continues.
So if you can create just enough “sinking in” to distribute pressure points over a wide enough surface area around the body prominences using comfort layers (and the upper surface of the middle layers if necessary) of various types and which will deal with the support needs of the recessed areas of the lumbar, then the only remaining issues are issues of alignment. This is controlled by the middle and deeper layers of the mattress and they need to prevent or “stop” any further sinking down of the heavier areas than are necessary (hips and pelvis primarily) to prevent them from sinking in further than necessary for pressure relief which could put the spine out of its natural range of good alignment.
In other words … the upper layers are about “allowing” enough sinking in to create the mattress cradle which is a larger area of surface contact with the mattress to relieve pressure and support the recessed areas of the spine. The deeper layers are about controlling any further compression than necessary for pressure relief to “hold up” the heavier parts of the body and keep the spine in its range of natural alignment. The amount of sinking allowed by lower layers has less effect on pressure relief in other words (in most types of mattress construction) but controls alignment. This is why when people change the firmness of upper layers to solve support issues or the lower layers to solve pressure relief issues they will often create new issues and may not solve the issue they are trying to “fix”.
Arms falling asleep often indicate a comfort layer that needs to be softer or thicker. The next most common reason is probably a pillow issue (likely needing a thicker or firmer pillow) which could also affect pressure on the shoulders. Of course the initial break in or adjustment period is also a possibility but this would generally lead to gradual improvement over time … particularly if you walked on the mattress to speed it up. The challenge of course is to figure out which it is so that any fine tuning or refinements can change things in the right direction. Sometimes people are sleeping better throughout the night and repositioning less often on their new mattress, and this is the cause of pressure build up.
The links contained in post #11 here provide quite a bit of good information about diagnosing possible reasons for should joint pain.
There is also more information in post #2 here that talks about the various symptoms that people may experience on a mattress and some of the reasons that can cause them that can help with the detective work or trial and error that may be necessary to identify the types of changes that may be helpful.
Hopefully that information is helpful to you.
Phoenix