What is the Origin of Melatonin Secretion Timing?

The term links 'melatonin' (from the Greek melas, black, and tonos, tension, due to its initial observation in skin darkening in amphibians) with 'secretion timing,' emphasizing the chronobiological aspect of its release. Melatonin's role as the "hormone of darkness" and a key marker of circadian phase has been a central focus of sleep and chronobiology research since the 1950s. The clinical importance of its timing, rather than just its quantity, is paramount.

What is the Mechanism of Melatonin Secretion Timing?

Melatonin secretion is rigorously regulated by the Suprachiasmatic Nucleus (SCN), the master clock, which sends inhibitory signals during the day and disinhibitory signals at night to the pineal gland. Light exposure to the retina, particularly blue light, activates the melanopsin pathway, which strongly inhibits the SCN's nocturnal signal to the pineal gland. This mechanism ensures that melatonin is only released when environmental darkness is detected, thereby setting the body's sleep-wake cycle.

Melatonin Secretion Timing

Meaning

The precise temporal pattern of melatonin release from the pineal gland, which typically begins in the evening, peaks in the middle of the night, and declines toward morning. This timing is a direct, reliable reflection of the body’s internal clock and serves as the primary hormonal signal for the onset of biological night. Disruption of this secretion timing, often measured by the Dim Light Melatonin Onset (DLMO), is a hallmark of circadian rhythm disorders.