I am one of those participating in the Cardiogram research (suffer from heart failure). I can tell when I am having PVC's (preventricular contractions - the ventricle pumps out of sequence thus future reducing the volume of blood being pumped to the body). My HeartWatch app will alert me that my heart rate has dropped below a preset level. I can generally feel it when awake, but I can see the results when I am asleep.