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Melatonin is an indole-derived signaling molecule, chemically N-acetyl-5-methoxytryptamine, with molecular formula ${\mathrm{C}}_{13}{\mathrm{H}}_{16}{\mathrm{N}}_2{\mathrm{O}}_2$. In mammals, the pineal gland secretes melatonin in a pattern related to environmental light and internal timing. The chemical identity is distinct from the time pattern of its synthesis and release.^[ref][ref]

The biosynthetic distinction is useful: serotonin N-acetyltransferase acetylates serotonin to form N-acetylserotonin; a later methyl-transfer reaction gives melatonin. Acetylation and methylation change different parts of the molecule, so detecting a precursor does not establish how much final product was made.^[ref]

A classic measurement examined timing upstream of the final product. D. C. Klein and J. L. Weller found that rat pineal N-acetyltransferase activity was more than 15-fold higher at night, and that a rhythm persisted in darkness or after blinding. Constant illumination suppressed it.^[ref]

For a hypothetical sampling example, measurements at four equally spaced times could give 1,4,1,4 units. Their mean is 2.5 units, also the mean of a constant 2.5-unit signal; the mean alone therefore cannot distinguish an oscillating signal from a steady one. This illustrates why timed measurements matter when studying a circadian rhythm. It does not turn the rat enzyme-activity ratio into a human blood-concentration ratio or a treatment-effect estimate.

The pineal gland is a small neuroendocrine organ of the epithalamus, attached to the roof of the third ventricle. It secretes melatonin, linking a neural timing system to a chemical signal distributed beyond the brain. Its anatomical position is distinct from that of the nearby habenula.^[ref]

A classic experiment located a strong daily rhythm at an enzymatic step in pineal metabolism. In 1970, D. C. Klein and J. L. Weller reported that rat pineal serotonin N-acetyltransferase activity was more than 15 times higher at night than during the day. The rhythm persisted in complete darkness and in blinded animals, whereas constant illumination suppressed it. Their experiments in isolated tissue also implicated norepinephrine in regulation of the enzyme activity.^[ref]

The darkness control is particularly illuminating. A mechanism that merely switched on whenever light was absent would predict a continuously high output in constant darkness. Continued rhythmic variation instead supports an internally generated timing signal. It does not show that the pineal gland alone generates the organism's entire circadian rhythm: the measurement concerns one gland and one metabolic activity in an intact timing system. Likewise, the reported activity ratio is an enzyme-assay result, not a universal ratio of human blood melatonin concentrations.