The 24 hour (diurnal) rhythm of our body such as the sleep-wake cycle is regulated by the hormone

Melatonin, also known as chemically N-acetyl 5-methoxytryptamine is a naturally occurring compound found in animals, plants and microbes. In animals, circulating levels of the hormone melatonin vary in a daily cycle, thereby allowing the entraiment of the circadian rythms of several biological functions.