A hormone tied directly to darkness
Melatonin is described as a hormone produced by the pineal gland specifically in response to darkness, with roughly eighty percent of the body’s daily output synthesised during nighttime hours. Blood concentrations swing considerably between night and day, running at roughly 80 to 120 picograms per millilitre at night compared with only 10 to 20 picograms per millilitre during the day, a difference driven by a direct light-sensing mechanism: when the eyes register sunlight, signals from the suprachiasmatic nuclei actively suppress pineal gland melatonin production, and that suppression is proportional to how intense and how long the light exposure is, with blue light in a specific narrow wavelength range identified as particularly effective at blocking production. This mechanism is what earns melatonin its description as the hormone of darkness, since its rise and fall track the light-dark cycle far more directly than any internal clock alone.
Discovered while chasing a skin treatment
The compound’s discovery traces to Aaron Lerner and colleagues at Yale University, who isolated melatonin in 1958 from extracts of bovine pineal glands, motivated originally by interest in potential treatments for skin disease rather than any specific curiosity about sleep. Melatonin synthesis itself proceeds through a defined chemical sequence starting from the amino acid L-tryptophan, converted step by step through intermediate compounds including serotonin before a final methylation step produces melatonin itself, a pathway that requires specific enzymes and cofactors along the way. The material notes that mitochondria have been identified as the main site of melatonin production within cells, a detail that connects the hormone’s synthesis directly to the same organelles responsible for a second, largely separate function discussed later.
Two guideline bodies, two opposite answers
As a sleep aid, the evidence for melatonin is presented as genuinely mixed rather than settled, and different guideline bodies have reached opposing conclusions from that evidence. Prolonged-release melatonin, marketed under the brand Circadin, has been licensed since 2007 for people aged 55 and over, and the 2023 European Insomnia Guideline recommends this specific prolonged-release formulation for up to three months in that age group, while explicitly advising against fast-release or over-the-counter formulations. The American Academy of Sleep Medicine’s 2017 guidelines take the opposite position, recommending against melatonin for insomnia treatment altogether, citing poor effectiveness and very low quality of evidence. This is a genuine, documented disagreement between major clinical bodies rather than a simple case of one guideline lagging behind newer evidence.
Where the evidence actually holds up better
Melatonin’s evidence base looks meaningfully stronger for certain circadian rhythm disorders than for ordinary insomnia. Meta-analyses conducted between 2005 and 2017 found melatonin improved sleep onset by around 40 minutes on average, and separately advanced the body’s own endogenous melatonin secretion timing by roughly 1.2 hours, effects that were notably larger in people with delayed sleep phase syndrome, where sleep onset latency improved by around 39 minutes, compared with people with ordinary insomnia, where the improvement measured only about 7 minutes. For jet lag, meta-analysis has judged melatonin probably effective, particularly for eastward travel, though the material notes that taking it at the wrong time can actually delay rather than speed adaptation, a caution that complicates any simple advice to just take melatonin before flying.
Not every proposed use is supported
Some proposed uses show little or no supporting evidence at all, and the material is direct about this rather than glossing over weaker claims. A 2020 Cochrane review found no evidence supporting melatonin for sleep problems in people with moderate to severe dementia, a clear limit on the supplement’s usefulness despite its popularity as a general sleep aid. For REM sleep behaviour disorder, melatonin is described as a safer alternative to the drug clonazepam, though clonazepam may actually be more effective, with the quality of evidence for both options rated very low. A more recent 2026 meta-analysis found melatonin supplements produced pain relief comparable in magnitude to typical analgesics, though again with modest effect size and low-to-moderate evidence quality, illustrating a broader pattern of melatonin showing some effect across several conditions without strong evidence behind any single one.
A second job entirely separate from sleep
Beyond its role in sleep, melatonin serves a second, largely independent function as a high-capacity antioxidant, a role recognised since 1993, working within mitochondria both by directly neutralising free radicals and by promoting the expression of genes for other antioxidant enzymes. This dual identity, sleep-timing signal on one hand and mitochondrial antioxidant on the other, is worth the hour precisely because most people encounter melatonin only as a supplement bottle promising better sleep, without ever learning that the guideline bodies most responsible for sleep medicine disagree about whether that promise holds up, or that the same molecule does entirely separate biochemical work inside the cell that has nothing to do with sleep at all.