When a resting heart beats less than 60 times per minute, it is a lower-than-normal heart rate, called bradycardia, states WebMD. A low heart rate can be a sign of fitness, as in athletes and healthy young adults, or a sign of a problem with the heart's electrical system.
In severe cases of bradycardia, the heart beats too slowly to supply the blood that the body needs to function properly, which can lead to a life-threatening condition, notes WebMD. Causes of bradycardia include heart changes due to aging, heart infections such as myocarditis and endocarditis, heart attacks, and diseases that injure the organ's electrical system. Medicines that treat heart problems or hypertension, such as beta-blockers and digoxin, a low thyroid level, or an imbalance of electrolytes can also cause a slow heartbeat.