Melatonin: far more than 'the sleep hormone'

Melatonin is first and foremost a hormone

Melatonin is not a vitamin, nor is it a classic sleeping pill. It is a hormone that our body produces naturally. The source we are most familiar with is a very small structure located in the brain: the pineal gland.

And this gland indirectly receives a piece of essential information: is it day or is it night? Because melatonin production is intimately linked to the natural alternation between light and darkness.

Your brain has a true internal clock

In our brain there is a small region called the suprachiasmatic nucleus. You don't need to remember the name; simply remember that it is one of the main clocks in our body. It coordinates our circadian rhythm — that is, the biological organisation of our day over approximately 24 hours.

This clock influences a great many things: sleep and wakefulness, body temperature, the secretion of certain hormones, metabolism, blood pressure, alertness, and numerous cellular functions. But to stay on time, it needs an external signal. And the most powerful of all is light.

Morning light is extremely important

When you expose yourself to natural light in the morning, specialised cells in your retina detect it and send the information to your biological clock. The message is simple: 'The day is beginning.' The production of melatonin by the pineal gland is then strongly suppressed.

This may seem paradoxical. Why would we want to reduce melatonin if our goal is to sleep better? Because we need a clear biological contrast between day and night. Daytime means light, activity, and alertness; night-time means darkness, rest, and melatonin. It is this alternation that matters.

To sleep better in the evening, sometimes look at what you do in the morning

This is a message I find particularly interesting. When someone sleeps poorly, their first question is almost always: 'What can I take tonight?' But another question deserves to be asked: 'What light did you receive this morning?'

If you wake up, stay in a rather dark house, then move into a car, and then spend the entire day under artificial light… your body is receiving a light signal very different from the one our physiology has adapted to over the course of evolution. Going outside in the morning and receiving natural light helps synchronise the circadian clock; and several hours later, when the light fades and evening arrives, the body is better prepared for its transition into night.

A word of caution: this is not 'the morning sun filling up your melatonin reserves'

I often like to use images to explain physiology, but one must know where the image ends. Morning light does not directly manufacture a reserve of melatonin that would be released twelve hours later. It acts primarily as a synchronisation signal for our circadian clock, and it is this clock — once properly calibrated — that then organises the nocturnal rhythm of melatonin secretion. The distinction may seem small; scientifically, it is important.

And in the evening, it is exactly the opposite

As light diminishes, the body gradually understands that night is approaching, and melatonin secretion begins to rise, reaching much higher concentrations during the night. But melatonin does not 'knock you out' the way a sedative would: it acts more like a biological night signal that informs the body that we have entered the nocturnal phase of the 24-hour cycle. That is why I sometimes prefer to describe it as the hormone of darkness rather than simply as the sleep hormone.

This is why evening light also matters enormously

We have created something very new in the history of humankind: a night that is no longer truly dark. Television, phone, tablet, computer, bright artificial lighting… we can send a light signal to our brain late in the evening, at a time when it should normally be receiving a darkness signal.

Nocturnal light, especially when it is intense and rich in certain wavelengths to which our circadian system is sensitive, can delay or reduce melatonin secretion. So, before thinking about a supplement, think about your light environment.

The much lesser-known part of the story

For a long time, we spoke mainly about the melatonin produced by the pineal gland. But we now know that melatonin, and the enzymes that enable its synthesis, also exist in many peripheral tissues: the digestive tract, the retina, certain immune cells, and various cellular compartments. And research is paying particular attention to its relationship with mitochondria. This is where the story becomes fascinating.

Melatonin and mitochondria

You have probably already heard of mitochondria. These are small structures present in most of our cells, which play a central role in energy production: they are involved in the synthesis of ATP, one of the principal forms of energy directly usable by the cell.

But producing energy is never entirely 'clean'. Mitochondrial function also generates highly reactive molecules — reactive oxygen species — which you probably know by a more familiar name: free radicals.

Are free radicals necessarily harmful?

No, and this is an important distinction. Our body uses certain reactive oxygen species as signalling molecules. The problem arises when a significant imbalance exists between their production and our defence capacities: this is what we call oxidative stress.

When it becomes excessive or chronic, this oxidative stress can contribute to damage to lipids, proteins, mitochondria and DNA. And this is precisely where melatonin possesses particularly interesting properties.

Melatonin is also an antioxidant molecule

Melatonin can help neutralise certain reactive species. But its interest does not stop there: it can also influence various antioxidant systems within our own body, such as glutathione, superoxide dismutase, catalase and other protective enzymatic systems.

In other words, it does not merely interact with a few oxidising molecules: it participates in an entire cellular defence environment against oxidative stress. And that is why it now attracts interest in many fields of research well beyond sleep alone.

What about red and near-infrared light?

This is undoubtedly the most fascinating part… and the one on which I want to be most cautious. Certain wavelengths of red and near-infrared light can penetrate tissues; they are studied in a field called photobiomodulation. We have experimental data showing that they can influence mitochondrial function and various cellular mechanisms, and some work even suggests a relationship between near-infrared light, mitochondria and local melatonin production. It is an extremely interesting avenue.

Can we therefore say that a red lamp causes your entire body to produce melatonin? Not yet, and this distinction is essential. We have plausible biological mechanisms and promising experimental data, but we do not yet have the same level of clinical evidence as for the darkness → pineal gland → circadian melatonin relationship. I am therefore not going to tell you 'use red light and your body will produce enough melatonin to slow your ageing': we are not there yet. Photobiomodulation is a serious and exciting field, but a promising hypothesis is not yet a clinical demonstration of longevity.

Melatonin = longevity?

Here again, caution is warranted. We know that quality sleep is essential to health, that a consistent circadian rhythm matters, that oxidative stress is involved in many mechanisms of ageing, that mitochondria play a central role in cellular physiology, and that melatonin has functions that go well beyond sleep. All of this makes its study in the field of ageing genuinely exciting.

But we cannot turn this equation into 'more melatonin = longer life'. Human longevity is infinitely more complex than that.

And with age?

Here is an interesting observation: nocturnal melatonin secretion tends to be lower in many older people, and, in parallel, sleep also changes with age — it often becomes more fragmented, lighter, earlier and sometimes less stable. But this does not mean that every person over 50 or 60 should automatically take melatonin: a physiological decline is not, in itself, an indication for treatment.

So, should you take melatonin to sleep?

Sometimes yes, but not necessarily for the reasons one might imagine. Melatonin is most useful when the problem involves the circadian rhythm: jet lag, delayed sleep phase, advanced sleep phase in certain contexts, or other circadian disorders. It may also have a role in certain cases of insomnia, depending on age, the type of formulation and the clinical context. But it is not a universal sleeping aid.

If you sleep poorly because of sleep apnea, chronic pain, significant anxiety, restless leg syndrome, depression, nocturnal reflux, or because you drink a lot of alcohol in the evening, taking melatonin does not address the underlying cause.

The dose matters… but the timing matters enormously

This is a fascinating particularity of melatonin. With many medications, we reason primarily in terms of dose; with melatonin, timing must also be considered. Taking it at a given hour can influence the circadian rhythm differently depending on the biological moment at which it is administered. This is why 'take melatonin before going to sleep' is sometimes a far too simplistic recommendation: if the goal is to shift a circadian clock, the time of intake can be decisive.

More melatonin is not necessarily better

Another common misconception: 'If 1 mg works, 10 mg will work ten times better.' No. Melatonin is a hormone, and the doses sold in certain supplements can far exceed what is sufficient to produce a biological signal. High doses can also cause, in some people, next-day drowsiness, very vivid dreams, headaches, dizziness or other adverse effects. The goal is therefore not to take the maximum dose, but to use the right strategy when an indication exists.

Can all supplements be trusted?

This is a genuine question. Analyses carried out on certain commercial products have shown that the actual quantity of melatonin can sometimes differ significantly from what is stated on the label, and both regulation and quality vary from country to country. Origin, formulation and quality therefore matter: a tablet labelled 'melatonin' is not necessarily equivalent to another.

Before the supplement, start with your biological clock

This is probably the most important practical piece of advice in this article. In the morning, expose yourself to natural light: ideally, go outside. Even on cloudy days, outdoor light is generally far more intense than the light inside your home. Walk for a few minutes, move around, tell your brain that the day is beginning.

During the day, stay active and try to receive natural light regularly, rather than staying cooped up in a dark environment. In the evening, gradually do the opposite: dim the light intensity, avoid very bright lights just before bedtime if possible, and reduce screen use when it is delaying your sleep, so as to gradually create an atmosphere that signals the end of the day. And at night, sleep in a room that is sufficiently dark, quiet, and at a comfortable temperature.

The modern paradox

We may have reversed the light environment to which our bodies had adapted. During the day, we stay shut inside relatively dim buildings; and in the evening, we light our homes as if it were the middle of the day. In other words: not enough light when our brain is expecting it, and too much light when it is expecting darkness. Then we sometimes try to repair that signal with a melatonin tablet. That, I believe, is where we need to change the way we think.

Key takeaways

Melatonin is not simply 'the sleep supplement': it is a hormone involved in the temporal organisation of our physiology. The melatonin produced by the pineal gland constitutes a biological signal of night; the light received by the eyes during the day, especially in the morning, helps to synchronise our circadian clock; and the darkness of the evening then allows our nocturnal physiology to settle in.

But melatonin also has functions that go beyond sleep: it interacts with mitochondria, oxidative stress, antioxidant systems, and numerous cellular pathways currently under study. As for red and near-infrared light, its interaction with mitochondria and possibly with peripheral melatonin is a fascinating area of research — but we still need to distinguish what we observe in the laboratory from what actually improves health and longevity in human beings.

I am Dr Florian Vallecillo, and if I were to leave you with just one idea today, it would be this: before looking for melatonin in a tablet, start by respecting the signals that allow your body to know when it is day and when it is night. Natural light in the morning, a truly active day, less light in the evening, a sufficiently dark night. And when supplementation is genuinely indicated: the right molecule, at the right dose, at the right time, and for the right reason. Because our sleep does not begin the moment we lie down: it begins, in a sense, with the very light our brain receives when we wake up.