melatonin
noun [ U ] chemistry, biology specialized uk/mel.əˈtəʊ.nɪn/ us/mel.əˈtoʊ.nɪn/
降黑素（体内的一种激素，能刺激肤色变化，与日常睡眠等生物节律有关）
a hormone in the body that produces changes in skin colour and is involved in controlling biorhythms such as our sleep pattern