Melatonin is a hormone which is secreted by the pineal gland. The secretion of its circadian rhythm is higher in the nighttime and lower in the daytime in characteristic. The physiological benefits of melatonin for humans are aiding sleep, reducing blood pressure, stimulating immune function, scavenging free radicals, enhancing antioxidative capacity, suppressing tumor necrosis factor and so on. The influence of exercise upon plasma melatonin may be related with the exercise time of day, circumstance illumination, exercise intensity and duration. At present we have known the aspect of single bout of exercise upon plasma melatonin as follows: exercise in the daytime elicits transient increases or keeps unchanged in plasma melatonin, but exercise at night blunts the nocturnal increase of plasma melatonin levels; As for the aspect of long term exercise: progressive exercise would rise plasma melatonin concentration, but vigorous exercise would diminish it. The future research can step toward the relevance of melatonin to overtraining to carry the new moment for sports training and sports health care.