Hi supermanx,
This is an interesting “sheet” phenomenon and based on speculation (I don’t know your height/weight or sleeping positions which would affect how the mattress feels to you) there could be one or more factors that are contributing to this.
The first is as you mentioned that the sheets are thick enough to prevent the amount of heat that is needed for the gel foam or the foam underneath it to reach its full “softness” potential.
The particles in gel memory foam are also meant to increase the support qualities of the memory foam (the beads make the foam firmer when it compresses more because they don’t displace like the memory foam) and that along with less heat and the 5 lb density foam underneath could be the reason you are feeling more softness in the upper couple of inches but then it becomes firmer as you sink in more. As you mentioned … it seems that you are not sinking in as far as you need/want to.
Next could be the depth of the sheet pocket of the fitted sheet. If this mattress is thicker and the sheets are fitting more tightly then it could be creating a drum like effect on the mattress.
Next could be the type of mattress protector or a mattress pad you may be using. Some are more or less stretchy and conforming than others and some allow more or less heat to go through them and there are many people who notice that changing their mattress protector or mattress pad they are using can make a real difference in the feel and softness of the mattress and the ability of the mattress to contour to the shape of your body (which can affect pressure points).
The type of cover or quilting in a mattress (or a topper) can also make a difference in how a memory foam layer responds for different people.
Upper back or shoulder and neck issues are often connected to a pillow as well and it’s not unusual at all for a new mattress to also need a new pillow because of the different way your body sinks in to the mattress and a pillow can be an important part of distributing weight in the upper body and maintaining good alignment.
The type of foundation or boxspring or other support system under the mattress could also make a difference (foam mattresses generally do best with rigid non flexing support underneath them).
Next could be the temperature and humidity of your room. As you know memory foam is heat and humidity sensitive and a cooler or cold room or less humidity can make the memory foam firmer.
Next could be the softening of the memory foam itself or the normal break in period for any new mattress as the memory foam will go through some initial softening (some more than others), covers will stretch a bit and lose their initial “stiffness”, and fiber materials will compress to a degree. Some types of higher quality/density memory foams take longer to go through their initial softening period to “break in” (some of the Tempurpedic HD mattresses are examples of this) and this can take up to 90 days or so although it’s normally 30 days or so and sometimes less. Walking on the mattress (covering the whole surface evenly) can help it to soften and break in a little quicker.
Next could be just the normal adjustment period that your body may need to adjust to any new sleeping surface. Like the break in period of a mattress, it can take some time to adjust to a new sleeping surface and for your body to “forget” it’s old sleeping habits and get used to a different mattress (see post #3 here). This is much like changing a “habit” where the body itself has cues and muscle memories that take time to change. This will usually take a month or less for most people but in some cases, like the initial break in period, it can take longer up to about 90 days or so. Because of this … it’s usually best to sleep on any new mattress for about 30 days or so or at the very least until it is clear that what you are experiencing is indicative of your long term experience on the mattress.
Many people are very surprised at how much their experience on a mattress can change over the first 30 - 90 days.
Finally (and this is less likely than the other possibilities when you have 5" of memory foam) the mattress may need a little more softer memory foam to “match” your body type and sleeping position (and slightly lower density memory foam that is a little softer could fit the bill here).
It’s difficult to say which of these it could be but it seems that at least the sheet (and possibly the protector/sheet combination) could be part of the issue.
Once you have ruled out the other possibilities mentioned here … the first step would be to talk with the retailer or manufacturer because they may also have some useful suggestions that are specific to their mattresses based on their experience with other customers that have purchased them as well.
If you can rule these out one at a time then the last option if necessary (outside of exchanging or returning the mattress) to add some additional thickness/softness and pressure relief to the comfort layers would be to add a topper to give you the extra softness and pressure relief you need and prefer. Post #2 here and the topper guidelines it links to has more information and sources for choosing a topper.
Phoenix