What’s in a mattress?
Who cares? The important fact is that a lot of this shit inside a mattress is designed to be somewhat fire retardant, especially in the US. That stuff that does not catch fire easily is excellent at retaining heat. It’s not air permutable, and it makes you sleep hot. It doesn’t mater who is the maker of the mattress. It’s the same for all of them.
Heat and moisture gets trapped because of the lack of circulation of air between the surface and your body. This, in addition to other factors, make you turn to a cooler spot on a mattress. Unconsciously. While you sleep. In order to allow air to circulate you need to adjust your mattress to permit air flow at certain places on a mattress. The “heat” locations are unique to your body. No manufacturer is aware of them.