Melatonin is a natural substance secreted by the pineal gland. Melatonin is recognized as a key element in regulating the body's day/night metabolic rates. Its secretion is synchronized with our biological clock which dictates the daily changes upon the body's metabolic activity: our circadian rhythms. Its most popular use is to help alleviate jet lag: Melatonin can alleviate jet lad tiredness after long haul flights (1). 1) Petrie, K, et al: Effect of Melatonin on jet lag after long haul flits. Brit. Med. J. 298, 705-7 (1989).