Melatonin is officially classified as a hormone. It is produced by the pineal gland in the brain and released into the bloodstream, where it travels to distant tissues throughout the body to regulate your sleep-wake cycle. That said, melatonin also acts locally in certain tissues like the retina, behaving more like a chemical signal between nearby cells. So while “hormone” is the correct and widely accepted label, the full picture is a bit more nuanced.
Why Melatonin Qualifies as a Hormone
The distinction between a hormone and a neurotransmitter comes down to three things: where the chemical is released, how far it travels, and how quickly it works. Hormones are produced in glands, secreted into the bloodstream, and act on distant organs or tissues. Their effects take minutes to hours to kick in. Neurotransmitters, by contrast, are released at the junction between two nerve cells and act almost instantly, within milliseconds, on the cell right next to them.
Melatonin checks every box for a hormone. The pineal gland, a small endocrine gland deep in the brain, synthesizes it and releases it directly into the bloodstream. From there it circulates throughout the body, reaching receptors in the brain, immune system, reproductive organs, and digestive tract. Its effects unfold over minutes to hours, not milliseconds. It is sometimes called the “hormone of darkness” because its secretion ramps up automatically when light fades and drops when light returns.
How Melatonin Works in the Body
Melatonin binds to two specific receptor types, known as MT1 and MT2, found on cells throughout the body. Both receptors work by reducing levels of a signaling molecule inside the cell called cAMP, which essentially dials down cellular activity. This is part of why melatonin promotes relaxation and sleepiness: it’s chemically telling your cells to quiet down.
Interestingly, MT1 and MT2 receptors can also pair up with serotonin receptors on the same cell. When melatonin binds in these paired arrangements, it can trigger different signaling pathways than it normally would on its own. This cross-talk between the melatonin and serotonin systems hints at why the two chemicals, which share a common precursor, influence overlapping aspects of mood, sleep, and cognition.
Once released, melatonin doesn’t linger long. Oral melatonin has a half-life of roughly 45 minutes, meaning half of it is cleared from your blood in under an hour. Your body’s own melatonin follows a similar pattern, which is why levels rise sharply after dark and fall well before morning.
Melatonin’s Local Signaling in the Retina
The reason this question comes up at all is that melatonin doesn’t only act as a long-distance hormone. In the retina, photoreceptor cells produce their own melatonin under the control of a local circadian clock, and that melatonin acts on neighboring retinal cells rather than traveling through the bloodstream. This local, cell-to-cell signaling looks more like what a neurotransmitter or neuromodulator does.
Research in mice has shown that melatonin directly increases the retina’s sensitivity to dim light. When melatonin was injected during the daytime, the retina responded to low light levels the same way it normally would at night, when natural melatonin is high. Mice that lacked the MT1 receptor showed no such boost, confirming that the effect depends on melatonin acting directly on photoreceptor cells. Melatonin also lowers the threshold for detecting faint light and amplifies the electrical signals that photoreceptors send to other retinal neurons. Because these effects happen locally, within the retina itself, scientists sometimes describe melatonin as a neuromodulator in this context.
So melatonin plays a dual role. Systemically, it is a hormone that coordinates your body’s daily rhythm. Locally, in tissues like the retina, it acts as a paracrine signal, meaning it influences the cells immediately around where it’s produced. Most textbooks and medical references still classify it simply as a hormone, and that’s the most accurate single-word answer.
From Tryptophan to Melatonin
Melatonin is built from serotonin, which itself comes from the amino acid tryptophan (found in protein-rich foods). The pineal gland converts serotonin into melatonin through two enzymatic steps, and the rate of this conversion is controlled by light exposure. When light hits your retina during the day, it sends a signal that suppresses melatonin production. After sunset, that brake is released and the pineal gland ramps up synthesis.
This shared ancestry with serotonin is one reason people occasionally confuse melatonin with a neurotransmitter. Serotonin is a classic neurotransmitter involved in mood, appetite, and pain. Melatonin is its downstream product, repurposed for an entirely different job: timekeeping.
What This Means for Melatonin Supplements
Because melatonin is a hormone, supplementing with it is fundamentally different from taking a vitamin or mineral. You’re introducing an external dose of a signaling molecule that your body already produces on a tightly controlled schedule. Short-term use appears safe for most adults, but long-term safety data is limited.
The hormonal classification also raises specific concerns for children and adolescents. Because melatonin interacts with the endocrine system, there is uncertainty about whether supplementation could affect hormonal development, including puberty timing, menstrual cycles, or production of other hormones like prolactin. These risks haven’t been confirmed, but they haven’t been ruled out either, largely because long-term studies in young people are scarce.
Major medical organizations remain cautious. The American Academy of Sleep Medicine and the American College of Physicians have both noted that evidence is insufficient to recommend melatonin for chronic insomnia. For delayed sleep-wake phase disorder, where your internal clock is shifted later than you’d like, the sleep medicine academy gave melatonin a weak recommendation in 2015, acknowledging uncertainty about whether its benefits outweigh potential harms. For people with dementia, the same organization recommends against melatonin use entirely.
None of this means melatonin supplements are dangerous. It means they deserve the same respect you’d give any hormone: they’re powerful signaling molecules, not inert sleep aids. Understanding that melatonin is a hormone, not a neurotransmitter, helps frame what you’re actually putting into your body when you take it.