Melatonin is a neurohormone synthesized from serotonin in the pineal gland, with production triggered by darkness and suppressed by light exposure. As a supplement ingredient, synthetic melatonin is bioidentical to the naturally produced hormone. Melatonin plays a central role in regulating the circadian clock, and supplementation has been extensively studied for reducing sleep onset latency and managing jet lag. Beyond its chronobiotic effects, melatonin is a potent antioxidant. Regulatory status varies by jurisdiction — it is sold as a dietary supplement in the US and China, while regulated differently in the EU.