Whenever I take a nap that is between fifteen minutes and two hours in length, I wake up with my heart racing. I am never dizzy or short of breath when this happens, but my entire body often shakes while my heart beats rapidly, similar to an adrenaline rush. The symptoms last for about ten minutes and do not seem to be correlated to times of stress or anxiety in my life, having been fairly consistent over the past few years. What causes this strange reaction to naps?