A common side effect of overtraining in athletes is chronic fatigue, which can manifest as prolonged tiredness and decreased performance. Overtraining occurs when training intensity and volume exceed an athlete's capacity to recover, leading to physical and psychological stress. Symptoms can also include elevated resting heart rate, irritability, sleep disturbances, and increased susceptibility to injuries and illnesses. It is essential for athletes to recognize the signs of overtraining and implement appropriate recovery strategies to maintain optimal performance levels and overall health.