Sinus bradycardia is defined as a heart rate of less than 60 beats per minute. Sinus bradycardia is common in many populations and does not usually require special treatment when it is caused by athletes, genetics or physical factors. In some people, sinus bradycardia is caused by sinus node malfunction, mainly in older people with degenerative sinus node degeneration or severe sinus bradycardia due to myocardial ischemia. If severe sinus bradycardia is less than 50 beats/min, it may lead to dizziness and darkness in the person.