Hi Sunshine4me,
This depends a lot on the overall construction and layers of the mattress. It is much more common with larger sizes (such as King size) where the center of the mattress is used much less and doesn’t compress as much as the areas of the mattress that are used more.
The main reasons for this are slight shifting of the materials in the mattress (towards the unused center), certain types of materials used in the upper layers that are more prone to compression than others (such as lower quality foams or various synthetic or natural fibers that will compress initially with use). The degree of these types of valleys will depend a lot on the type of layering and construction of the upper surface of the mattress but in general latex and other higher quality foams are less prone to softening and impressions than either fiber (usually in the mattress quilting) or lower quality foam that will develop soft spots or ridges more quickly.
There are many retailers that will say a mattress is “latex” when this is just one of the layers in the mattress and there are other materials above the latex which they may not tell you about. this is particularly true with some of the larger brands where you may unknowingly be sleeping directly on fairly thick layers of polyfoam without realizing it.
You are somewhat “out of the way” in Logan but some of the better options or possibilities in the Salt Lake City area I’m aware of are listed in post #2 here. The closest of these that sell “foam” mattresses (memory foam and latex) is R & S Mattress and Crown Elite bedrooms in Ogden.
I did a quick search for retailers that are more local to you if this is too far and some of the better possibilities in the Logan area (and the brands they carry I would look at on a “mattress by mattress” basis and some comments about the few I talked with) are …
http://www.thatsmymattress.com/ Logan, UT. Southerland, Pure Latex Bliss, Bed Boss
http://www.palmerhomefurnishings.com/vendors.html Logan, UT Restonic, Cannon
Redirecting... Logan, UT. Talked with Nathan here and they carry Restonic on the floor (latex hybrids and gel foam) and also have access to Bountiful rest for custom builds. Their Restonic factory is Everton mattress. He has good knowledge about materials and is well worth visiting.
http://www.theroomloft.com/ Brigham city, UT. Carries SleepInc (Spring Air) and Diamond (made in Salt Lake City). Talked with Julie. Has a Diamond innerspring hybrid with 2" Talalay for $699 queen (very good value) and others as well.
When you are working with or talking to local retailers that haven’t been “fully researched” … it’s important to make sure that they are knowledgeable about mattress materials and are able to provide you with the quality specs of all the layers of any mattress you are considering.
Hope this helps
Phoenix