Bradycardia is a resting heart rate of under 60 beats per minute. It is rarely symptomatic until the rate drops below 50 BPM, however, it can result in fatigue, weakness, dizziness, 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 tachycardia during training.