Monday morning. Rain outside. Three meetings before lunch. Someone has already replied-all to an email that absolutely did not require replying-all. Yet your colleague walks in smiling, makes coffee, chats pleasantly about the weekend and seems psychologically prepared to face another spreadsheet.
You, meanwhile, are considering whether disappearing into a forest is still a realistic career option.
It is tempting to blame serotonin.
Perhaps your colleague simply has more of it. A naturally luxurious supply. Premium-grade serotonin, delivered directly from some cheerful little factory in the brain.
There is a grain of truth here. Serotonin biology really does differ between people. Genetics, stress, inflammation, diet, hormones, medications and other factors can affect how serotonin is produced, transported, broken down and received by neurons.
But the popular idea that happiness can be reduced to a serotonin gauge marked EMPTY to FULL is badly outdated.
Modern neuroscience paints a stranger and far more interesting picture.
So what exactly is serotonin?
Serotonin, technically called 5-hydroxytryptamine or 5-HT, is a chemical messenger used throughout the body.
In the brain, serotonin acts as a neurotransmitter and neuromodulator. That second word is important. Serotonin does not simply switch feelings on and off. It changes how neural circuits behave.
It influences sleep, appetite, learning, memory, pain, sexual behavior, emotional processing, stress responses, patience, impulsivity and decision-making. A recent review describes at least 14 serotonin receptor subtypes belonging to seven receptor families, which already gives us a clue that we are dealing with something more complicated than a biological happiness button. Different receptors can produce very different, sometimes opposing, effects.
Most serotonin-producing neurons in the brain originate in relatively small structures called the raphe nuclei in the brainstem. From there, their fibres spread widely through the brain.
Recent research has also weakened the old picture of serotonin neurons as one homogeneous army doing roughly the same thing everywhere. Different populations of serotonin neurons appear to participate in different emotional and behavioral circuits, sometimes producing quite different responses.
So asking, “How much serotonin does this person have?” is already a little like asking, “How much traffic does this city have?”
Where? At what time? Moving in which direction?
And is anyone actually getting anywhere?
Your gut makes most of your serotonin, but there is a catch
Here comes one of serotonin’s favourite pieces of trivia: roughly 90 to 95 percent of the body’s serotonin is produced outside the brain, predominantly in the gastrointestinal system.
This has helped create a small industry of claims suggesting that fixing your gut will somehow send large quantities of happiness directly upstairs.
Unfortunately, biology has installed a border checkpoint.
Serotonin circulating in the bloodstream does not readily cross the blood-brain barrier. Brain serotonin and peripheral serotonin are therefore largely separate systems. A blood serotonin measurement cannot simply tell you how much serotonergic activity is occurring inside your brain.
The gut can still influence the brain, just by more indirect routes.
The microbiome can affect immune signalling, metabolism of serotonin’s precursor tryptophan, vagal signalling and other parts of the gut-brain communication network. A 2025 Nature Reviews Microbiology review describes this relationship as a genuinely bidirectional biological system, but not as a simple pipeline carrying gut serotonin into the brain.
Reality has once again refused to fit neatly on a wellness influencer’s infographic.
How does your brain make serotonin?
The process begins with tryptophan, an essential amino acid that we obtain from food.
Inside serotonergic neurons, tryptophan is converted into 5-hydroxytryptophan and then into serotonin. One particularly important enzyme is tryptophan hydroxylase 2, or TPH2, the form responsible for serotonin synthesis in neurons.
A 2025 review of serotonin metabolism emphasizes the distinction between TPH1, which operates mainly in peripheral tissues, and TPH2, which performs this job within the nervous system.
That gives us our first reason why serotonin biology may differ between you and your suspiciously cheerful colleague. The machinery is not identical.
Genetics sets some of the controls
Genes influence enzymes, receptors and transport proteins involved in serotonin signalling.
Researchers have investigated variations in genes including TPH2, which helps synthesize serotonin; SLC6A4, which produces the serotonin transporter; MAOA, involved in serotonin breakdown; and genes controlling serotonin receptors themselves.
A PET imaging study of 140 healthy adults, for example, found associations between certain genetic variants and measurable differences in serotonin-transporter availability in several brain regions. At the same time, the genetic effects were hardly simple enough to turn DNA into a serotonin horoscope.
This is an important correction to an older fashion in psychology. For years researchers hoped that individual genes, especially variants of the famous serotonin-transporter gene, might neatly explain vulnerability to depression or stress.
Large modern genetic studies have made that story much less convincing. Depression and emotional traits are highly polygenic. Thousands of biological influences interact with experience and environment. One gene rarely gets the honour of ruining your Tuesday afternoon all by itself.
Food matters, but not in the way advertisements suggest
Because serotonin begins with tryptophan, it sounds logical that eating more tryptophan should produce more serotonin.
Turkey contains tryptophan. Eggs contain it. Cheese contains it. Nuts and seeds contain it. So perhaps the road to happiness really is a sufficiently determined cheese sandwich?
Tryptophan has to cross the blood-brain barrier, and it competes with several other amino acids for transport. What matters is therefore not merely how much tryptophan you eat, but its availability relative to competing amino acids, together with the metabolic conditions inside the body.
A major 2025 review of nutritional influences on the serotonergic system concluded that nutrition can influence serotonin synthesis and metabolism, but through a complicated network involving tryptophan availability, vitamins, minerals, fatty acids and metabolic pathways.
Nutrition matters, but it refuses to act like pharmacology.
Stress and inflammation may change where tryptophan goes
Tryptophan does not exist merely to make serotonin. In fact, most of it travels through another biochemical route called the kynurenine pathway.
Inflammation can increase activity along this pathway. Immune signals stimulate enzymes that send more tryptophan toward kynurenine metabolism, potentially altering the amount available for serotonin production while generating several biologically active compounds of their own.
Research increasingly connects this system with interactions between chronic stress, inflammation and psychiatric illness. A 2024 review describes how inflammatory signalling can alter tryptophan metabolism and produce kynurenine metabolites that affect glutamate signalling and neural function.
This does not mean that inflammation simply “uses up your serotonin and causes depression.” That would merely replace one simplistic story with another.
It means that immune activity, stress hormones, neurotransmitters and brain circuits interact. Someone living under prolonged stress or inflammatory conditions may therefore develop different serotonin-related biology from another person even if their genes and diets are similar.
Sleep, light and daily rhythms matter too
Serotonin is tied to circadian biology, and daylight has long been associated with seasonal changes in mood.
Older studies suggested that serotonin turnover varied with sunlight exposure. That helped produce the attractive idea that winter simply lowers serotonin.
But newer evidence makes the story less tidy.
A 2026 meta-analysis examining serotonin-transporter availability in healthy people found no significant overall seasonal change when the strongest longitudinal studies were considered. Earlier apparent winter increases in serotonin-transporter binding were driven largely by cross-sectional studies.
That does not mean winter, light exposure, or circadian disruption are irrelevant to mood. They clearly matter.
It means we should be careful about turning “winter affects mood” into “winter empties the serotonin tank.”
Exercise is another example
Exercise improves mental health for many people, and serotonin has often been proposed as one reason. Yet even here the simple explanation is shaky.
A 2026 systematic review and meta-analysis examined randomized trials measuring circulating serotonin after exercise. It found no statistically significant overall increase, although the available research was small and highly variable. More importantly, the authors stressed that blood serotonin is not a reliable measure of serotonin activity in the brain.
Exercise may still affect serotonergic circuits. It also changes inflammation, neuroplasticity, stress hormones, sleep, cardiovascular function, dopamine signalling and dozens of other processes.
Apparently the human body neglected to appoint a single Department of Happiness.
More serotonin does not automatically mean more happiness
This may be the biggest misconception of all.
In 2024 researchers experimentally increased synaptic serotonin in humans and studied what happened to behavior. The results were interesting.
Increasing serotonin altered aversive learning and increased behavioral inhibition and impulse control. Participants became less sensitive to outcomes in unpleasant situations and showed changes in how they responded to negative emotional information.
Notice what the researchers did not discover: “Everyone became happier.”
Serotonin appears to help regulate how organisms respond to uncertainty, punishment, threat, reward and competing impulses. Depending on the receptor, brain region and situation, increased serotonin signalling may encourage patience, inhibition or emotional flexibility.
So two people could theoretically produce similar amounts of serotonin while responding quite differently because their receptors, transporters and neural circuits differ. And two people with different rates of serotonin synthesis might nevertheless behave remarkably similarly.
This is why neuroscience increasingly talks about serotonergic signalling, rather than treating serotonin concentration as a psychological fuel gauge.
What about depression?
This is where decades of popular science have left the largest mess. For many years depression was commonly explained as a “chemical imbalance”, especially a shortage of serotonin.
The modern evidence does not support such a simple model.
A widely discussed umbrella review found no consistent evidence that depression is generally caused by low serotonin concentration or reduced serotonin activity. That conclusion itself has been challenged by other researchers, who argue that the review oversimplified parts of the evidence and that serotonin clearly remains involved in depression biology. The scientific disagreement is important. What both sides increasingly reject is the crude equation: low serotonin = depression.
A Molecular Psychiatry review proposed a broader framework based around neuroplasticity, arguing that antidepressant effects cannot adequately be understood as simply replacing missing serotonin.
This also explains an apparent paradox.
SSRIs increase serotonin availability relatively quickly by blocking its reuptake. Yet their clinical effects often take considerably longer to develop.
The important changes may involve downstream adaptations in receptors, neural networks, emotional processing and plasticity rather than the initial increase in serotonin itself. The tap moves first, the plumbing takes longer.
So why might your serotonin system differ from your colleague’s?
Probably because almost everything biological differs slightly between two people.
You may have different versions or expression levels of enzymes involved in serotonin synthesis and breakdown. Different numbers or sensitivities of serotonin receptors. Different serotonin-transporter activity.
Your history of stress, your sleep, diet and metabolism, immune system, and medications may differ. Your brain developed under different conditions and spent decades adapting to different experiences. And crucially, there may be no single meaningful answer to the question of who has “more serotonin.”
Your colleague could have greater serotonin synthesis in one region but stronger re-uptake in another. You could have lower serotonin release but more sensitive receptors. Particular serotonin circuits might react more strongly to stress in one person and barely respond in another.
The comparison quickly starts to resemble two orchestras where the instruments, acoustics, musicians, and conductor are all slightly different.
Your brain is a regulator, not a chemical soup
The more neuroscience learns about serotonin, the less useful the old chemical-balance metaphor becomes.
Serotonin undoubtedly matters. Changing serotonergic signalling can change human behavior. Genetics influences the system. Nutrition, immune activity, stress and medications interact with it. Different brains genuinely operate differently.
But serotonin does not determine whether you are happy, miserable, confident, anxious, patient or irritable. It participates and that distinction matters.
Your suspiciously cheerful colleague may indeed have different serotonin biology. But they also have different receptors, different neural networks, different experiences, perhaps eight hours of sleep and possibly the enviable ability not to read work email after 6 p.m.
There are limits to what neurotransmitters should be blamed for.
We are not walking serotonin measurements. Human moods and personalities emerge from a living system that is constantly adjusting itself to genes, bodies, relationships, memories, environments and experience. Which admittedly makes us harder to explain.
But considerably more interesting than a chemical tank with a smiley face printed on the side.
