= Melatonin
\b[Melatonin] is an indole-derived signaling <molecule>, chemically N-acetyl-5-methoxytryptamine, with <molecular formula> $\mathrm{C_{13}H_{16}N_2O_2}$. 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.<Melatonin in PubChem>[CID896: formula and structure.]{ref}<Pineal Gland in NLM MeSH>[D010870 scope note: neuroendocrine organ and melatonin secretion.]{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.<Enzyme Nomenclature by the IUBMB>[EC2.3.1.87 Comments and linked EC2.1.1.4: penultimate acetylation and subsequent O-methylation.]{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.<Indole metabolism in the pineal gland: a circadian rhythm in N-acetyltransferase by D. C. Klein and J. L. Weller>[Science 169,1093–1095(1970), abstract: activity endpoint and lighting controls.]{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.
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