Hi gme109,
I would keep in mind that stiffness can come from lying in a position where your spine, joints, or muscles arenāt fully relaxed and in their neutral position for an extended period of time and this can be the result of a mattress that is too firm or a mattress that is too soft. Both can lead to alignment issues and stiffness (and if this is only an issue that happens sometimes and not others then it could also be related to your activities during the day as well). It may be worth trying either firmer top comfort layers (say medium over soft) or firmer transition or support layers (say soft over firm over medium) to see how your experience changes so you can ālearnā from how your experience and symptoms change with firmness changes in comfort and support layers that go in different ādirectionsā.
There isnāt a way that can confirm this āscientificallyā without specialized equipment (and there is also some debate about the type of equipment and testing methods that do the best job for this because itās so complex). Some of the better sleep studies on posture and alignment and sleep ergonomics that Iāve seen had to design their own equipment and methodology ā¦ see the study here that includes the following quote
ā¦
This book is one of the best Iāve read and has two chapters devoted to measuring and assessing spinal alignment in a lab (scroll down to the description of chapter 3 and chapter 4).
Different people will have different āneutralā postures and physiology so good alignment can sometimes ālook differentā for different people but from a consumer perspective the most reliable way to assess alignment would be based on āeyeballsā which compares your alignment when you are standing up straight with good posture with your sleeping posture and based on your actual experience and āsymptomsā when you sleep on a mattress (see this article and post #11 here).
Phoenix