Because of its low resilience, memory foam is not suited to be a support layer
It is so important to pay attention to the layers underneath the memory foam in a mattress as this is where the deep support (ability to keep your spine aligned) comes from. Treating memory foam as “supportive” as opposed to “pressure relieving” will usually lead to a potential purchaser paying less attention to underlying parts of the mattress and their resilience and other qualities, and possibly choosing a mattress with a poor ability to provide good alignment for a their body, weight distribution and/or different sleeping positions. Because a lot of the more heat-sensitive memory foam on the market can allow you to keep sinking in further over the course of the night as it softens, the underlying parts of the mattress that will prevent your heavier parts from sinking in so far that your spine is out of alignment are also important and some mattresses that have thicker layers of memory foam may keep you in good alignment when you first go to sleep at night but you may be out of alignment when you wake up in the morning.
Some side effects of memory foam’s greater sensitivity to heat can also lead to sleeping issues for some people
The deeper in a mattress someone sleeps, the more likely they are to have issues with “sleeping hot.” This of course is a quality of all foams where you “sink in” to some degree but it is compounded by the makeup of memory foams in general which allow a greater degree of sinking in and are typically less breathable (allow for less evaporation) than other foams. Even the newer generation memory foams which are more breathable, are not as breathable as other foams that are readily available. The greater breathability of other foams which have a more “open” cell structure (like latex or other polyfoams) tend to lessen the heat issues even for those who like to sleep more “in” a mattress using softer “non memory” foams. Natural fibers breathe best of all and tend to be cooler than any foam … especially memory foam.
This same sensitivity to heat can also lead in some cases to a mattress becoming “too soft” or “too hard” depending on the external temperature
The temperature in your bedroom can change its feel from season to season or from what you experienced in the store depending on environmental conditions. Different types of memory foam can be more or less sensitive to this but it can be more important to control the temperature of your bedroom with some memory foams than it is with others or with other materials.
Another potential issue of memory foams is that they take more time to adjust to different positions.
This can be an issue for those who change positions often or are sensitive to the time it takes for the memory foam to conform to their new position as it can create short term “pressure” while it forms a new “cradle”. Again different types of memory foams will take shorter or longer to conform to a new position. This “time to compress” or “rebound” that changes with temperature is both part of memory foam’s strength for some (creates a “stable cradle”) and its weakness for others (doesn’t conform to new positions quickly enough or feels too firm). Some people may also be sensitive to a lack of resiliency or “pushback” which allows them to change positions more easily with a little “help” from the mattress and helps to support the lumbar area. This same lack of resiliency or “springiness” is also why it is often rated lower than other materials for the “other activities that take place on a mattress”.
Finally there are the “offgassing issues” of some of the poorer quality foams that are common in the market today.
While all memory foams and polyurethane foams in general (including the “green” ones) use some “nasty” materials in their manufacture, some of them have more of this material left in them by the time you sleep on them than others. For those that are sensitive to this offgassing, this can lead to issues ranging from a reaction to the unpleasant smell itself all the way to respiratory issues caused by the vapors. Your best protection against this is to make sure you know who manufactures the memory foam used in a mattress and not just accept the “re-branded name” that has been given to it (and to you). If the foam in your mattress has been certified by Certiur (or a similar organization), you can be reasonably sure that at least any smell or offgassing that you may notice has been tested for any potential harm it may cause you (within the limits of the test). More natural materials used in mattresses such as different fibers and good quality latex foam (which may also have a less unpleasant odor for a short time and is usually tested as well) are usually considered to be superior in this area. In the case of CertiPur certification, the foam will also have been tested to some degree for durability and so is less likely to lose its beneficial qualities in a few months after purchase but the density of the memory foam would still be much more important durability factor than any limited durability testing done by CertiPur.
Without knowing what specifications and certifications the memory foam in your mattress has, I would not buy it.
While it’s not directly connected to the quality of memory foam itself because higher heat and humidity levels can speed up the softening and break down of any foam material … with more temperature sensitive materials like memory foam this can happen faster and so I would tend to avoid using memory foam with heated mattress pads or blankets or at the very least use them at the lowest setting and only for short periods of time.