Bradycardia is a heart rhythm disorder that involves a slow heartbeat. A normal heart rate is between 60 and 100 beats per minute. It is called bradycardia when the rate falls below this standard. During sleep, bradycardia is natural. It can be caused by taking medications which result in slowing the heart rate such as beta-blockers, a compression of the carotid sinus, or certain heart conditions. Drug treatment can restore a normal rhythm, but sometimes severe bradycardia requires the installation of a cardiac stimulator or pacemaker.