To quote our article on sleeping positions…
“Back sleeping is the second most common position and has a slightly less curvy profile than side sleeping. The “gap” that needs to be filled in with the comfort layer for pressure relief is generally the small of the back (the lumbar area) and because the “gaps” in your profile are not quite as deep as in side sleeping, a slightly thinner top layer will generally work a little better. In back sleeping in other words a slightly shallower cradle is needed. A good starting point for a back sleeper is 2” of softer material on top of your mattress and then increasing or decreasing from there depending on other factors like weight, preferences, or other sleeping positions."
Since you’re new to mattress shopping, I’d also recommend you take a look at our Mattress Shopping Tutorial and this article about durable materials.
Unfortunately, memory foam very easily collapses under body heat which is why it tends to “sleep hot.” However, there’s a lot to be said about the microclimate of a mattress.
There are also some types of synthetic fibers (such as coolmax) which are specifically designed to draw moisture away from the skin and disperse it to the rest of the layer which can be effective as well.
In terms of sheets and bedding … natural or artificial fibers (cotton or various cellulosic fibers) are more cooling than synthetic fibers and linen is perhaps the coolest of all.
You can see more about the different factors that can affect sleeping temperature in post #2 here and more about the microclimate of a mattress in post #29 here.
I hope this helps! We are here to answer any questions 
NikkiTMU