My reading on this board suggests that latex is less likely to retain body heat because of vertical air channels through the rubber. I would guess that body warmth is probably dependent on (1) bedding materials, (2) mattress casing materials, (3) mattress composition, and (4) mattress cushiness.
- Bedding materials: some sheets and mattress pads naturally retain more heat than others
- Mattress casing materials: well, they can differ in thickness and textile; some natural-fiber advocates can be vocal on the subject
- Mattress composition: specifically the parts which you're sleeping directly on (batting, polyurethane foam, latex, memory foam)
- Mattress cushiness: if you've sunk 3" into a soft mattress, the same materials will communicate/reflect more heat to you than a firmer mattress
Memory foam softens in response to heat, so your warmer bits sink further in. Memory foam is also a poor conductor of heat away from your body, so the warmed and softened memory foam will stay close to you throughout your nap. This is less true for latex, although we've been sleeping on a rather soft latex layer lately and I've found it a bit warm (but then I tend to feel warmer at night than my wife does, and it has been warmish lately). We're about to switch to a firmer latex mattress and I expect it to be a bit cooler. Before we bought this mattress we had an innerspring mattress, but it was enclosed in a vinyl bag due to my allergies and the vinyl reflected some warmth too.