aluminized mylar will reflect radiant heat back to your body, BUT - it only reflects radiant heat; (not your biggest form of heat loss); it works poorly at even that unless at an optimum distance for your body - which as I understand it is something like 1/2-3/4" of an inch. Further or closer it loses efficiency pretty quickly. So on the ouside of a sleeping bag (unless it is a damn thin one) it will do very little. On the inside of a sleeping bag, it will do little unless it is performing as a vapor barrier (as others have pointed out). On top of your foam pad, where your sleeping bag is compressed to almost nothing, it will not do much.
In an emergency, where you use it on the outside of your clothing (which may well be in that optimum thickness range) it can be pretty handy, especially as it also blocks the wind. Radiant barriers have been tried in sleeping bags a few times in the past: most successful ones I experienced were made by Moonstone Mountaineering back in the early 80's,(my retail days) using a perforated aluminized mylar. they put a 1/2" thick layer of polarguard on the inner side of the mylar, them more polarguard outside of it . It worked pretty well, but experience showed that the aluminized coating was not durable enough - it washed off and wore off - so they dropped the idea. I had a friend who worked at Moonstone, who got me some of the mylar, and I made a bag out of it with just a nylon fabric lining, and slept in it in my typical summer backpacking clothes down to about freezing. Again, my clothes were just about the right thickness to keep the reflective surface at the optimal distance. So it is a useful concept in some very limited circumstances.