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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.