All personal heating devices, such as heat pads, come with a warning about proper use to avoid burning yourself. Indeed, a superficial burn is actually quite likely if the heating pad is not used correctly.
For example, the heating pad should not be used continuously on the highest heat setting. Nor should anyone sleep on top of one. It's too easy to either lie too long in one position, or to bunch up the pad, creating an intense heat spot. This can easily cause a second degree burn that includes blisters.
But there is another side effect of extended use of heating pads that is never mentioned. I call it the Radiator Effect.
To understand what that means, we must consider what happens in your car's radiator. Why is there a radiator in the first place? Simply put, without it, the car would overheat, cause engine damage and render the engine useless.
The concept of a radiator is that a reservoir contains water and coolant; this fluid is circulated, using a water pump, through a system of hoses and waterways in and around the engine to keep it cool enough to function without damage. To keep this fluid in the optimum temperature range, a thermostat controls the flow. If the engine starts to get too hot, the thermostat allows more fluid to circulate. But a certain amount of heat is desirable, too, so when it's not warm enough yet, the thermostat restricts the flow until things warm up. This is accomplished with a little valve in the thermostat that controls the flow.
The body works in a similar fashion, though the physiology isn't exactly the same, of course.
When a person lies down on a heating pad, a warming effect is felt. As blood flows through the muscles near the heat source, it also warms up, carrying the additional warmth throughout the body. This leads to a relaxing effect and provides relief for sore muscles, for example. But it happens precisely because applying heat to one area is actually not normal and the body signals the blood vessels to open wide and circulate more blood to carry the heat away from that location. It's a built-in method of protection. Sort of like a radiator.
However, the benefit is short lived if allowed to continue too long, and the opposite effect is created instead... that is, the muscles become more stiff and sore.
This occurs because the body cannot allow itself to become overheated. Therefore, the blood vessels in the vicinity of the heat source tend to narrow, and the capillaries may even close off circulation to specific areas to avoid picking up more heat. This is desirable only to the point of preventing overheating, but it's temporary. The discomfort one might feel is the warning to remove the heat source.
But if you're sleeping, you probably won't feel the signal to turn off the heat. This is when a burn is most likely to occur and you will wake up with a red spot at best, or a blister, along with stiff and sore muscles. The affected muscles have been deprived of proper circulation because the body was protecting itself from too much heat... a radiator effect, essentially.