Overview
Melatonin is heavily marketed as a natural sleep aid and anti-aging compound, but its real function is to imitate winter and nighttime in the body. It causes regression of the thymus gland and gonads, lowers body temperature, suppresses progesterone and thyroid, and raises estrogen. The enzyme that produces melatonin in the pineal gland converts serotonin into melatonin under the trigger of adrenaline at night, so melatonin is fundamentally a detoxification product of serotonin. Most of the favorable research that drove melatonin's reputation since the 1990s was done in rats and mice, which are nocturnal animals whose melatonin works in the opposite direction from how it works in humans. Physiological doses are in the microgram range, while typical store-bought 5 to 10 milligram supplements are at least a thousand times higher and produce wintertime endocrine effects.
Key Points
-
Melatonin's primary function is to detoxify serotonin, not to cause sleep. The enzyme activated by adrenaline in the pineal gland converts serotonin into melatonin. Adrenaline rises in darkness because ordinary light prevents some of the free radical activity that increases at night. Melatonin is a detoxification product downstream of stress and serotonin, and in general it functions as an antidote to serotonin throughout the body. Inflamed breast tissue and rheumatic joints contain the same enzyme system, which is the body's attempt to clear local serotonin rather than evidence that melatonin is causing the inflammation.
-
Melatonin imitates winter and triggers regression of the thymus gland and gonads. In all animals from fish, turtles, lizards, rats, and sheep, the reproductive system shrinks during winter. The thymus gland regresses too, which is part of why people are more susceptible to infections in winter. Melatonin is the main thing that causes this involution. The original pineal extract studies were done before melatonin was even confirmed as the main pineal hormone and showed effects that imitated nighttime and winter, including seasonal affective disorder patterns of depression and weight gain.
-
Most of the positive melatonin research was done in nocturnal rodents and is opposite to what happens in humans. In humans and pigs, both daytime species, melatonin suppresses progesterone, suppresses thyroid, and raises estrogen. In rats and mice, which work at night, melatonin raises prolactin, and prolactin raises their progesterone, which gives pro-life and anti-aging effects in those species. Almost all of the anti-aging claims for melatonin are based on this rodent data and do not transfer to humans. The studies on melatonin's reproductive effects in pigs by the researcher Sirotkin showed it suppresses progesterone and raises estrogen, the same pattern produced by low thyroid.
-
The physiological dose of melatonin is in micrograms, not milligrams. One published estimate puts the physiological dose at about 5 micrograms for an average-sized adult. Standard 5 to 10 milligram supplements are at least a thousand times physiological in terms of acute exposure, which is precisely why they reliably produce sleep: they overwhelm normal regulation. Antioxidant and radiation-protection studies often use a thousand to a million times physiological concentrations to get their effects.
-
Long-term melatonin supplementation lowers progesterone and thyroid while raising estrogen. In otherwise healthy animals, melatonin alone is capable of lowering both progesterone and thyroid hormone while increasing estrogen. This is the same hormonal pattern produced by hypothyroidism, which is why high melatonin reproduces many low-thyroid symptoms: low temperature, sluggish metabolism, immune suppression, fertility problems.
-
Melatonin lowers body temperature, which is associated with worse outcomes in sick patients. Lowering body temperature drops immune function and tissue repair capacity. Hospitalized patients with the lowest temperatures are the least likely to survive. The body uses melatonin to lower temperature during hibernation, and forcing that pattern on yourself with supplements creates a similar suppressed metabolic state.
-
The good side of melatonin is real but limited to short-term protection in a stressed brain. In a fatigued, de-energized brain where nitric oxide rises and creates a vicious cycle through acetylcholine activation, melatonin is effective at turning off that cycle and protecting against over-excitation. Very large doses given during or shortly after radiation exposure are protective against gamma ray damage. These are targeted, time-limited interventions, not a basis for chronic daily supplementation.
-
The conversion of serotonin to melatonin depends on ATP, not just enzyme activity. Healthy people convert serotonin to melatonin because they have the cellular energy to drive the reaction. The enzyme is a cofactor; the rate-limiting input is ATP. This is why hypometabolic people, whose ATP production is low, end up with serotonin pooling in tissues and melatonin running low at night.
-
Sunlight, especially near-infrared, drives endogenous melatonin production at the cellular level. Around 95 percent of melatonin in the body is produced subcellularly in the mitochondria in response to near-infrared light, with only about 5 percent coming from the pineal gland at night. Roughly 70 percent of solar radiation is in the near-infrared spectrum, which is why direct sunlight exposure during the day, particularly around solar noon, supports melatonin production far better than supplementation does. Blue light suppresses melatonin synthesis, depletes dopamine, and accelerates serotonin synthesis.
Notable Quotes
"It can disturb your progesterone and thyroid production if you take a lot of it"
[Ray Peat — Hormone Replacement Therapy (KMUD, April 2013)]
"Melatonin is the main thing that causes involution (shrinking) of the thymus gland and gonads"
[Ray Peat — Mitochondria, GABA, Herbs (KMUD, April 2016)]
"If you take 10 milligrams of melatonin, then that's going to be a problem, but a milligram or so is probably harmless"
[Ray Peat — Serotonin and Endotoxin]
"If you take melatonin, it has basically a reverse feedback, negative feedback effect. It's going to raise your serotonin."
[Georgi Dinkov — Mercola Interview on Estrogen and Serotonin]
Important Things To Consider
Standard supplement doses are at least a thousand times physiological. Even a 1 milligram tablet is several hundred times what the body normally produces. If melatonin is being used at all, microgram-range tablets (100 to 300 micrograms) are far closer to physiological. The pharmacological effect of overwhelming the system is exactly why high-dose supplements work for sleep, and also why they create endocrine disruption with chronic use.
Melatonin is not a solution to insomnia caused by adrenaline. Most insomnia, especially the kind that wakes people up at 3 a.m. with a pounding heart, is driven by hypoglycemia and the resulting adrenaline surge. The actual fix is to keep blood sugar steady through the night, with a small carbohydrate snack at bedtime if needed. Melatonin masks the adrenaline spike but doesn't address the metabolic problem causing it, and over time it suppresses thyroid in a way that worsens the underlying picture.
5-HTP is much more dangerous than melatonin. 5-hydroxytryptophan is sometimes marketed alongside melatonin as a natural sleep aid, but 5-HTP turns massively into serotonin, which suppresses immunity, stimulates cortisol, and damages eyes. People prescribed both fish oil and 5-HTP have presented with cataracts and glaucoma. If somebody is choosing between serotonin-pathway supplements, melatonin is the safer of the two, but neither is recommended.
Don't interpret melatonin in inflamed tissue as the cause of the inflammation. Italian researchers have found high melatonin in rheumatoid arthritis patients and concluded melatonin is causing the disease. The opposite is true: inflamed tissue activates the same pineal enzyme system to clear local serotonin, which is the actual driver of inflammation. The melatonin found in inflamed breast tissue or arthritic joints is the body's attempt at detoxification, not the source of the problem.
A normally functioning thyroid produces normal melatonin without supplements. When thyroid is active, melatonin rises as an antioxidant defense against the higher metabolic rate. When thyroid is low, melatonin is also low. Supplementing melatonin to compensate for low-thyroid sleep problems doesn't fix the underlying thyroid state and creates additional endocrine problems on top.
Avoid melatonin if you're trying to support reproductive function, immunity, or recovery. The endocrine signature of melatonin in humans is winter, hibernation, gonad shrinkage, and thymus involution. Anyone trying to conceive, dealing with infertility, recovering from surgery, fighting infection, or wanting strong immune function should avoid chronic melatonin supplementation. Living near the equator, where seasonal melatonin shifts are minimal, is associated with the absence of winter-pattern immune disturbance.
Large doses can be useful during or shortly after radiation exposure. The antioxidant properties are real at very high concentrations, and protection against gamma rays is documented. This is a narrow, time-limited use, not a daily routine. Outside of acute radiation events, the wintertime endocrine pattern outweighs the antioxidant benefit at chronic supplemental doses.
Taking SSRIs poisons the melatonin pathway. SSRIs raise serotonin to levels high enough to block the enzyme that converts serotonin into melatonin, which is part of why people on SSRIs sleep poorly. The official explanation blames depression, but the direct cause is the elevated serotonin itself. The state SSRI users are in resembles hibernation more than sleep: light, non-restorative, and easily disrupted.
Endotoxin in the gut is upstream of the whole problem. Most of the body's serotonin is produced by enterochromaffin cells in the intestine, and endotoxin from gut bacteria activates these cells. Keeping bowel transit fast and frequent, and avoiding endotoxin-producing foods, lowers peripheral serotonin production at the source, which lets melatonin synthesis proceed normally without supplementation.