How our organism understands, what it is time to wake up?

On accumulation in it certain substances the Cycle of a dream and wakefulness is regulated by several systems. "chemical hours" are their cornerstone — a chain of the biochemical reactions interfaced so that accumulation of some substances slows down its further synthesis. In such system maxima and minima of concentration of it of substances alternate through constant periods. At cellular level the daily rhythm is regulated mainly by PER and TIM proteins. For interpretation of this mechanism Nobel Prize was awarded 2017. And at the level of an organism a similar role the hormone plays melatonin...