So anyway, here goes. Heat should only be applied for 20 to 30 minutes at a time. Again, more is not always better. This is one of those times. Here is why.
Heat does several things to the body.
It increases the circulation and it effects the nervous system. You get on your heating pad and it feels great, so you stay on it for several hours. You feel pretty good while you re on it. Then you take the heating pad off and you notice your muscles are sore and stiff, so you get back on it for several more hours. This turns into a vicious cycle.