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Bradycardia is a condition typically defined wherein an individual has a resting heart rate of under 60 beats per minute (BPM) in adults, although some studies use a heart rate of less than 50 BPM. Bradycardia typically does not cause symptoms until the rate drops below 40 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.[citation needed] Highly trained athletes may also have athletic heart syndrome, a very slow resting heart rate that occurs as a sport adaptation and helps prevent tachycardia during training. The term 'relative bradycardia' is used to refer to a heart rate that, although not actually below 60 BPM, is still considered too slow for the individual's current medical condition. [from Wikipedia]
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