This is a problem with memory foam mattresses in general, they trap too much heat. It's not too bad if you live in a cold climate, but I live in a hot climate and during the summers it got to be too much.

Gel memory foam is the nice compromise. You get the memory foam but with a gel topper. It's noticeably cooler and a bit firmer than typical memory foam. We got one and have since moved the regular memory foam into the guest room.