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Bradycardia is a condition wherein an individual has a slow heart rate, typically defined as a heart rate of under 60 beats per minute (BPM) in adults.  Bradycardia typically does not cause symptoms until the rate drops below 50 BPM. When symptomatic, it may cause fatigue, weakness, dizziness, sweating, and at very low rates, fainting. 
During sleep, a slow heartbeat with rates around 40–50 BPM is common, and is considered normal. Highly trained
athletes may also have
athletic heart syndrome, a very slow resting heart rate that occurs as a sport adaptation and helps prevent
The term relative bradycardia is used in explaining a heart rate that, although not actually below 60 BPM, is still considered too slow for the individual's current medical condition.