Melatonin is the hormone of darkness.

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Melatonin production tracks the day–night cycle: levels rise in the dark and fall with light, helping signal to the body that it’s time to sleep. The pineal gland makes melatonin under the control of the brain’s circadian clock, which responds to light information from the eyes. When it’s dark, sympathetic signals stimulate the pineal cells to convert serotonin into melatonin, promoting sleepiness. When light—especially blue light—enters the eyes, those signals are dampened, and melatonin production decreases, helping wakefulness resume. Because its nocturnal rise is so closely tied to darkness, it’s commonly called the hormone of darkness. So the statement reflects a well-established relationship between darkness and melatonin’s role in signaling night and sleep.