Hi BSON,
Memory foam will soften in response to heat, temperature, and the length of time it is continuously compressed. The changes are more rapid over the course of the first few minutes but there can be smaller changes over time as well so with thicker layers of memory foam there is a greater risk that you may be in good alignment when you first go to sleep at night but may be out of alignment when you wake up in the morning. How much this may affect any specific person on a specific memory foam mattress will depend on the design of the mattress, the thickness of the memory foam layers, and on the specific formulation of the memory foam(s) in the mattress.
When you are testing memory foam mattresses I would tend to spend a little extra time with your testing compared to the normal 15 minutes I would suggest for other types of mattresses to give the memory foam a little more time to warm up.
There is more about the pros and cons of memory foam in general in this article.
There are also hundreds of different formulations of memory foam on the market that have different firmness ranges and different properties (such as slower or faster response times, more or less breathable, more or less temperature sensitive etc) depending on the chemical formulation of the memory foam. There is more about the different properties that can be formulated into of different types of memory foam in post #9 here and in post #8 here.
Phoenix