Why am I exhausted all day, then suddenly wide awake at night? Why can I finally concentrate when everyone else has gone to bed? And why can one late night make the next day feel disproportionately difficult?

It turns out ADHD isn’t only linked to differences in attention. Research increasingly points to differences in sleep timing and the internal body clock too, with studies finding greater “eveningness”, later sleep onset and delayed circadian rhythms in people with ADHD.

One recent review reported sleep disturbances in up to 80% of adults with ADHD. Even more interestingly, one of the biological markers scientists use to measure the body clock has been found to occur around 90 minutes later in some adults with ADHD.

Quick recap: what is your circadian rhythm? (1,2)

Your circadian rhythm is your internal 24-hour clock. It helps coordinate when you feel alert and sleepy, as well as melatonin release, body temperature, hormone timing and the wider sleep-wake cycle.

One way researchers measure this is through DLMO, or dim-light melatonin onset. Put simply, this is the point in the evening when your body starts increasing melatonin, signalling that biological night is beginning.

In adults with ADHD, that signal can arrive later. A systematic review of 62 studies, including 4,462 people with ADHD, found consistent evidence of later chronotype, delayed sleep onset and shifts in circadian markers including melatonin timing. (1)

So it is possible to feel physically exhausted while your internal clock is still saying: not bedtime yet.

ADHD sleep may be later — and less stable (3)

A 2026 study followed 54 adults with ADHD and 47 adults without ADHD for 10 days. Participants wore activity trackers, allowing researchers to observe their sleep and daily activity patterns rather than relying only on questionnaires.

The ADHD group had later and less stable daily rhythms, greater eveningness and more sleep difficulties. Researchers also found that taking longer to fall asleep was associated with poorer selective attention, while shorter sleep was associated with poorer inhibitory control.

Some of these associations weakened after stricter statistical correction, so the study cannot tell us that poorer sleep directly causes worse ADHD symptoms. But it adds to a growing body of research suggesting that sleep timing and daytime cognition are closely connected.

Why can bedtime be so difficult with ADHD?

“Go to bed” sounds like one task, but it is actually a chain of them. You have to notice the time, stop whatever you are doing, disengage even if it is interesting, remember everything that needs doing before bed, move through several low-reward tasks and then transition from stimulation to lying still.

That requires a lot of stopping, switching, remembering and task initiation, all areas that can already create more friction for someone with ADHD.

Then there is another factor: night can finally be quiet. The emails stop, meetings are over, fewer people need something from you and there are fewer interruptions competing for your attention. After spending the day responding to other people's priorities, 10.30pm may be the first point at which your attention finally feels like your own.

So instead of wanting to sleep, you want to use it.

Why can ADHD brains suddenly feel more awake at night? (4)

Part of the answer may lie in the relationship between the circadian system and dopamine.

Dopamine is involved in motivation, reward, attention and learning, and differences in dopamine signalling have long been part of ADHD research. Circadian biology, meanwhile, looks at how brain and body systems change across the 24-hour day.

Increasingly, researchers are studying the two together. A 2026 review concluded that dopamine signalling and circadian timing appear to interact in both directions: dopamine can influence circadian timing, while the circadian system can also influence dopamine-related pathways. (4)

This doesn’t mean we fully understand the relationship yet, but it raises an interesting possibility. If circadian timing shifts later, some of the brain systems involved in attention, motivation and reward may also be operating differently depending on the time of day.

In other words, your brain at 9am is not biologically identical to your brain at 11pm.

The ADHD sleep mismatch

This becomes particularly relevant when your biological timing and your actual life do not match.

Your internal clock may favour a later night, but work still starts at 9am. Children still wake up early. The school run does not move because you fell asleep at 1am.

That means a later body clock does not necessarily result in simply sleeping later. Often, it results in sleeping less.

Research in adults with ADHD and sleep-onset insomnia has found delayed melatonin timing and later sleep periods compared with controls. So while biological night may shift later, the required waking time often stays exactly where it is.

Can poor sleep make ADHD feel worse?

Potentially. Sleep loss affects many of the same cognitive functions that can already be difficult in ADHD, including attention, working memory, inhibitory control, decision-making and emotional regulation.

This can create a fairly obvious loop: it is difficult to stop at night, sleep begins later, morning still arrives at the same time, the next day takes more effort, and you arrive at the following evening tired but not necessarily ready to switch off.

Then it starts again.

But does improving sleep improve ADHD? (5)

This is where the research becomes more nuanced.

A 2026 trial looked at 70 adults with ADHD who also screened positive for a sleep disorder. Researchers compared ADHD treatment alone with approaches that also targeted sleep.

Treating sleep improved sleep quality and fatigue, but it did not significantly improve overall ADHD symptoms compared with ADHD treatment alone in the main analysis. (5)

That distinction matters. Sleep can be important without being the explanation for everything, and improving it does not mean ADHD simply disappears.

The relationship appears to work more like overlapping systems than one straightforward cause and effect.

So what does the research actually tell us?

ADHD and sleep are strongly connected, but the relationship is not as simple as “ADHD causes bad sleep” or “better sleep fixes ADHD”. Research points to differences in circadian timing, greater evening preference, difficulty initiating sleep and, in some people, a mismatch between the timing their body prefers and the timetable their life requires.

It also raises a broader question that is easy to overlook: how much of tomorrow begins the night before?

We often talk about focus as a daytime problem and sleep as a nighttime problem, but the brain does not divide itself that neatly. How easily you stop at night affects how much sleep is available before morning, and the condition you wake up in affects the attention, inhibition and emotional bandwidth available for the next day.

Morning and night are not separate systems. They are two parts of the same 24 hours.

References
Coogan AN, McGowan NM. A systematic review of circadian function, chronotype and chronotherapy in attention deficit hyperactivity disorder. Attention Deficit and Hyperactivity Disorders. 2017.
Luu B, Fabiano N. ADHD as a circadian rhythm disorder: evidence and implications for chronotherapy. Frontiers in Psychiatry. 2025.
Walter A, Martz É, Weiner L, et al. Sleep, rest-activity rhythm, cognitive and emotional symptoms in adult ADHD: unraveling the links with an actimetry-based approach. BMC Psychiatry. 2026.
Glavan D, Udristoiu I, Hermann DM, et al. Chronobiological disruption in ADHD: Developmental mechanisms linking sleep, behavior, and comorbid conditions. Progress in Neuro-Psychopharmacology & Biological Psychiatry. 2026.
van der Ham M, Bijlenga D, Molenaar N, et al. The Effects of Sleep Treatment on Symptoms of ADHD, Sleep Quality, Fatigue, and Depressive Symptoms in Adults. Journal of Attention Disorders. 2026.

This journal is for general education and is not medical advice.

There are an estimated 38 trillion bacteria living in the human body, roughly the same order of magnitude as the number of human cells.

Most of them live in your gut.

But the interesting part isn’t how many are there. It’s what they are doing.

Gut microbes break down parts of food we cannot digest ourselves, produce metabolites that interact with our immune system, influence the intestinal barrier and communicate with nerves running between the gut and the brain.

In 2025, scientists went one step further. In mice, they identified a neural circuit capable of detecting a molecule produced by gut bacteria and relaying that information towards the brain.

Your gut, it turns out, is much more than a digestive tube.

First: what exactly is your microbiome? (1)

Your microbiota is the collection of microorganisms living in and on you, including bacteria, viruses, fungi and other microbes. The microbiome includes those microorganisms along with their genes, products and surrounding environment.

The largest microbial community lives in the gastrointestinal tract, particularly the colon.

One widely cited calculation estimates that a typical 70 kg adult male contains around 38 trillion bacterial cells and 30 trillion human cells. The exact number varies enormously between people, but it corrected an older myth that bacteria outnumber our own cells by 10 to 1.

The ratio is actually much closer to one to one.

So are we half bacteria?

By cell count, roughly.

By mass, definitely not. Those bacteria together are estimated to weigh only around 200 grams.

What are all those bacteria doing? (2,3)

Some of the most important work happens when your microbes get access to food that your own digestive enzymes cannot completely break down.

Dietary fibre is a good example.

Certain gut bacteria ferment fibre and produce molecules called short-chain fatty acids, predominantly acetate, propionate and butyrate.

These are not simply waste products.

Butyrate is used extensively by cells lining the colon, while short-chain fatty acids can interact with immune cells, receptors, metabolism and intestinal barrier function. A major 2025 review in Nature Reviews Microbiology describes them as microbial metabolites with effects extending well beyond the bacteria that produced them.

This means something quite strange is happening every time you eat fibre.

You do not possess all the enzymes necessary to extract everything from it yourself. Instead, some of what you eat becomes food for your microbes, and what they make from it can then interact with you.

It is less like feeding one organism and more like feeding an ecosystem.

So how can something in your gut communicate with your brain? (4)

The phrase gut-brain axis can make this sound slightly mystical.

It isn't.

There are several physical communication routes between the gut and brain, including the nervous system, immune signalling, hormones and molecules produced or modified by gut microbes.

One of the major neural routes is the vagus nerve, which carries sensory information from internal organs towards the brain.

The gut lining also contains specialised sensory cells capable of detecting what is happening inside the intestine. Some sit remarkably close to sensory nerve endings, creating a route by which information in the gut can be translated into a nervous-system signal.

A major 2025 review described this interface between microorganisms, intestinal sensory cells and nerves as an emerging biological system through which microbial information can reach the nervous system.

And then researchers actually watched one of these circuits work.

In 2025, scientists found what they called a “neurobiotic sense” (5)

This is one of the most interesting recent findings in gut-brain research.

Researchers publishing in Nature studied flagellin, a structural protein found in the flagella used by many bacteria.

They found specialised sensory cells in the mouse colon capable of detecting this microbial signal.

Those cells then released PYY, which activated neurons connected to the vagus nerve. The signal travelled towards the brain and changed feeding behaviour.

When researchers disrupted the system, the mice ate more and gained more weight.

The scientists called it a “neurobiotic sense”: a sensory mechanism through which the animal could detect a molecular pattern associated with the microorganisms living inside it.

This was a mouse study, so we cannot assume exactly the same mechanism controls human behaviour.

But mechanistically, it is important.

Gut-to-brain communication is no longer just an abstract idea based on associations between microbes and mood. Researchers can now trace specific sensory cells, receptors and neurons involved in transmitting microbial information.

Does your gut really make serotonin? (6)

Yes.

But this is also one of the most misunderstood facts in microbiome science.

Around 95% of the body's serotonin is produced outside the brain, predominantly in the gastrointestinal system, much of it by specialised enterochromaffin cells in the intestinal lining. Gut microbes and microbial products can influence this production.

This has led to the often repeated claim that your gut “makes your happiness hormone”.

That is not quite how it works.

Serotonin produced in the gut does not cross the blood-brain barrier and simply become serotonin inside your brain. Peripheral and brain serotonin largely operate as separate pools.

Gut serotonin still matters enormously. It is involved in intestinal movement, secretion, sensation and other signalling processes. The microbiome can also affect tryptophan metabolism and other pathways that may influence communication between the gut and nervous system.

But “95% of serotonin is made in the gut” does not mean “95% of your happiness comes from your gut”.

The real biology is more interesting than the slogan.

Your microbiome can change surprisingly quickly (7)

Your microbiome is partly shaped over years, but parts of it can respond to what you eat remarkably fast.

In a well-known controlled study published in Nature, researchers gave participants diets made entirely from either animal or plant foods and tracked what happened to their gut microbes.

The microbial community responded within days.

On the animal-based diet, bile-tolerant species increased while several bacteria involved in breaking down plant polysaccharides decreased. Microbial gene activity also changed substantially.

So the microbiome is not a fixed collection of organisms you acquire in childhood and keep forever.

It is an ecosystem constantly responding to its environment.

Food is part of that environment.

So are medication, illness, age and many other aspects of daily life.

Does eating more fibre automatically create a “better” microbiome?

Not necessarily, and this is where microbiome science gets more interesting.

A Stanford randomised dietary study compared diets high in fibre with diets high in fermented foods in healthy adults over 10 weeks.

The fermented-food group showed an increase in microbial diversity and reductions across a panel of inflammatory proteins. The high-fibre group, unexpectedly, did not show an overall increase in microbial diversity over the same period.

That does not mean fibre is unimportant. Fibre remains fundamental to gastrointestinal and wider health, and different fibres feed different microbial communities.

Instead, the study showed that microbiomes do not all respond identically to the same intervention.

Researchers also found that participants' starting microbiomes appeared to influence how they responded to increased fibre.

Which brings us to one of the biggest problems with the phrase “healthy gut”.

We still don't know what the perfect microbiome looks like (8)

There probably isn't one.

Two healthy people can have remarkably different collections of gut microbes.

A major 2025 Nature Reviews Microbiology paper involving an international group of microbiome researchers examined exactly this question and concluded that defining one universally “healthy” microbiome remains extremely difficult.

Microbial communities vary by person, geography, age, diet, medication, environment and over time. Different bacterial communities can also perform overlapping biological functions.

So a microbiome test telling you that one particular bacterium is “low” does not necessarily mean something is wrong.

And simply having more microbial diversity does not automatically equal better health in every context.

What the microbes do may ultimately be as important as exactly which names appear on the list.

Can changing the gut microbiome actually change cognition? (9)

This is where the evidence becomes particularly interesting, but also where we need to separate promising research from established fact.

In 2024, researchers studied 36 pairs of twins aged 60 or older in a double-blind randomised controlled trial.

One twin received a daily prebiotic containing inulin and fructo-oligosaccharides, while the other received a placebo. Both groups also received protein supplementation and an exercise programme.

After 12 weeks, the prebiotic changed the gut microbiome, including an increase in Bifidobacterium.

The researchers also found a statistically significant improvement in a combined cognitive score in the prebiotic group compared with placebo. There was no significant improvement in the trial's primary physical outcome.

It was a relatively small study in older adults, so it would be a big leap to claim that prebiotics generally make people smarter.

But it matters because this is human randomised evidence, not simply an observational link between certain bacteria and certain behaviours.

The next question is whether findings like this can be reproduced in larger groups, across different ages, with different fibres and with clearly defined mechanisms.

What about the microbiome and ADHD? (10)

This area has grown quickly, but the results are nowhere near as settled as social media sometimes suggests.

A systematic review published in June 2026 examined 23 human studies comparing the gut microbiota of children with and without ADHD.

Most studies found no significant difference in alpha diversity, meaning the overall diversity within an individual's microbiome was not consistently lower or higher in ADHD.

Differences in beta diversity, which looks at how microbial communities differ between groups, appeared more frequently. Some bacterial patterns also appeared repeatedly, including lower abundance of certain butyrate-producing bacteria such as Faecalibacterium.

But results were inconsistent between studies and affected by differences in diet, age, geography, medication, sequencing methods and other potential confounders.

So there is a signal worth studying.

There is not yet an “ADHD microbiome”.

And there is currently no convincing evidence that changing one particular bacterial species treats ADHD.

That distinction is important.

The gut-brain axis works both ways

We normally hear about the gut influencing the brain.

The brain also influences the gut.

Stress, autonomic nervous system activity, intestinal movement, secretions and eating behaviour can all alter the environment microbes live in. Recent mechanistic research has even shown brain circuits capable of rapidly changing gut microbial composition in mice.

So the gut-brain axis is not:

gut → brain

It is:

gut ↔ brain

with the immune system, nervous system, diet, microbial metabolites and hormones sitting between them.

This is why reducing everything to “good bacteria” and “bad bacteria” misses most of the interesting biology.

So what do we actually know?

We know the human gut contains an enormous microbial ecosystem. We know microbes metabolise parts of our diet that our own enzymes cannot, producing compounds that interact with intestinal, metabolic and immune systems. We know the gut has direct neural connections with the brain, and animal experiments can now trace some microbial signals through specific gut sensory cells and neurons.

We also know diet can change microbial activity quickly, that different people respond differently to the same foods, and that human trials are beginning to show measurable effects of microbiome-targeted interventions outside the gut.

What we do not yet have is a universal definition of the perfect microbiome, a single bacterial profile that explains cognition or ADHD, or evidence that manipulating one microbe gives predictable effects in everyone.

The science is moving quickly.

The most interesting shift may be a fairly simple one: the gut is no longer being studied as an organ that just processes food.

It is being studied as an ecosystem, an immune interface, a metabolic organ and a sensory system communicating continuously with the rest of the body.

And we are only beginning to understand the conversation.

References
Sender R, Fuchs S, Milo R. Revised Estimates for the Number of Human and Bacteria Cells in the Body. PLoS Biology. 2016.
Mukhopadhya I, Louis P. Gut microbiota-derived short-chain fatty acids and their role in human health and disease. Nature Reviews Microbiology. 2025.
Sanz Y, Cryan JF, Deschasaux-Tanguy M, et al. The gut microbiome connects nutrition and human health. Nature Reviews Gastroenterology & Hepatology. 2025.
Ohara TE, Hsiao EY. Microbiota-neuroepithelial signalling across the gut-brain axis. Nature Reviews Microbiology. 2025.
Liu WW, Reicher N, Alway E, et al. A gut sense for a microbial pattern regulates feeding. Nature. 2025.
Banskota S, Ghia JE, Khan WI. Serotonin in the gut: Blessing or a curse. Biochimie. 2019; together with subsequent reviews of gut-derived serotonin signalling.
David LA, Maurice CF, Carmody RN, et al. Diet rapidly and reproducibly alters the human gut microbiome. Nature. 2014.
Joos R, Boucher K, Lavelle A, et al. Examining the healthy human microbiome concept. Nature Reviews Microbiology. 2025.
Ni Lochlainn M, et al. Effect of gut microbiome modulation on muscle function and cognition: the PROMOTe randomised controlled trial. Nature Communications. 2024.
Rodrigues B, Miranda IM, Costa de Oliveira S. Gut Microbiota Composition and Diversity in Attention-Deficit/Hyperactivity Disorder: A Systematic Review. Microorganisms. 2026.

This journal is for general education and is not medical advice.

You have probably heard the simple version.

Probiotics add “good” bacteria. Prebiotics feed them.

That is broadly true, but it misses the more interesting difference.

A probiotic introduces live microorganisms into an ecosystem that is already home to trillions of microbes. Those new organisms have to survive manufacturing, storage, your stomach, bile and then competition from the microbial communities already living there.

A prebiotic takes a different route. It provides a substance that certain microorganisms already in your body can use.

And increasingly, research suggests that who is already living in your gut can influence what happens next.

First: what actually counts as a prebiotic? (1,2)

Probiotics are officially defined as live microorganisms that, when given in adequate amounts, confer a health benefit.

The word live matters.

Prebiotics are different. The scientific definition is “a substrate that is selectively utilised by host microorganisms conferring a health benefit”.

In normal language: something reaches your microbes, particular microorganisms can use it, and that interaction has a demonstrated benefit for the person taking it.

This also clears up a common misconception:

Not every fibre is a prebiotic.

Many fibres are fermented by gut bacteria, but to technically qualify as a prebiotic, there needs to be evidence of selective microbial use linked with a health benefit.

So “fibre” and “prebiotic” are not interchangeable.

Your gut isn't empty space waiting for new bacteria

The idea behind probiotics sounds intuitive: if certain microorganisms are associated with health, why not simply add more of them?

The complication is that your gut is already occupied.

In a landmark human study, researchers gave participants an 11-strain probiotic combination and then looked not only at stool samples, but directly at different areas of the gastrointestinal tract.

The response varied dramatically between people.

Some were more permissive to colonisation. Others resisted it. Whether individual probiotic strains established themselves depended partly on the person's existing microbiome and features of their gut.

The researchers also found something particularly interesting: detecting probiotic organisms in somebody's stool did not necessarily tell them whether those organisms had successfully colonised their gut lining.

So taking billions of bacteria does not necessarily mean billions of bacteria move in.

Some probiotics may still have effects while passing through the gastrointestinal tract. Permanent colonisation is not required for every probiotic benefit.

But your resident microbiome is not passive.

It can resist newcomers.

Prebiotics take a different approach

Instead of trying to introduce a new organism, prebiotics work through microorganisms already present in the host.

Certain microbes have enzymes that allow them to break down particular carbohydrates that our own digestive enzymes cannot fully use.

When that material reaches the colon, bacteria can ferment it and produce other compounds.

So rather than:

new bacteria → gut

the idea is closer to:

substrate → resident bacteria → microbial activity

And it turns out that what bacteria do may be just as important as which bacteria are there.

Because bacteria don't just eat your food. They feed each other. (3)

Your microbiome is not a collection of individual organisms working independently.

Microbes compete for nutrients, exchange metabolites and use the products made by other species.

This is called cross-feeding.

A 2024 study in Nature Microbiology showed just how complicated this can become.

Researchers were studying what gut microbes do with tryptophan, an amino acid obtained through food. Different bacteria can turn tryptophan into very different metabolites.

When researchers introduced fermentable fibre into the microbial community, fibre-degrading bacteria released sugars that could then be used by other bacteria. That changed the competition for tryptophan and changed which metabolites were ultimately produced.

Importantly, fibre didn't simply make one “good bacterium” grow.

It changed microbial behaviour.

That is a much more accurate way to think about the microbiome.

Sometimes you are feeding one organism.

Sometimes you are feeding the organism that feeds another organism.

And sometimes that changes what the whole community produces.

What do your bacteria actually make from fibre? (4)

Some of the best studied microbial products are short-chain fatty acids, particularly acetate, propionate and butyrate.

They are produced when gut microorganisms ferment certain dietary carbohydrates.

Butyrate is particularly important locally because it can be used as an energy source by cells lining the colon. Short-chain fatty acids also interact with receptors, immune pathways and metabolic processes.

A major 2025 review in Nature Reviews Microbiology describes an increasingly complex picture in which the effects depend on which short-chain fatty acid is produced, where it is produced, how much is available and the wider biological context.

So “fibre feeds good bacteria” is not wrong.

It is just several chapters too short.

You eat the substrate. Your microbes metabolise it. Their metabolites then interact with you.

And the same prebiotic may not do the same thing in every person

This may be one of the most important changes happening in microbiome research.

In December 2025, researchers published a randomised multicentre study involving 802 people with prediabetes. Participants received either usual care or a substantial dietary fibre intervention for six months.

When everyone was analysed together, there was no significant difference between the fibre and control groups in the main metabolic outcomes.

But then researchers found something much more interesting.

Some groups responded to the fibre intervention while others did not, and differences in their gut microbiomes were associated with those responses.

Researchers developed a model using participants' microbiomes at the beginning of the study to predict who was more likely to respond to the fibre intervention. They then tested the model against data from two other fibre studies.

This does not mean we can currently sequence everyone's microbiome and prescribe their perfect fibre.

The analysis was partly post hoc, the population had prediabetes and the cohorts were geographically specific.

But the underlying idea is important:

The effect of fibre can depend on the ecosystem it arrives in.

Another study found the same fibre could meet two very different guts (5)

A 2025 double-blind randomised trial studied healthy adults taking inulin, resistant-starch-rich unripe banana flour or a control.

Before the intervention, researchers identified two broad microbiome patterns among the participants: one richer in Prevotella and another richer in Bacteroides.

The groups did not respond identically.

The strongest microbiome changes following the resistant starch intervention appeared in participants belonging to one of those starting microbiome groups.

Again, this is early research.

But it explains why the future of microbiome nutrition may look very different from:

Everyone take the same probiotic.

Your starting point matters.

One prebiotic fibre being studied particularly closely is PHGG (6)

Partially hydrolysed guar gum, or PHGG, is a water-soluble fermentable fibre produced by breaking longer guar gum molecules into smaller chains.

A 2024 randomised, double-blind crossover study followed 33 healthy adults through periods in which they received 3g PHGG, 6g PHGG or placebo.

After 14 days, both PHGG doses were associated with significantly greater levels of Akkermansia than placebo. The researchers also saw changes in other microbial groups, including a decrease in Faecalibacterium at the 3g dose, showing why describing microbiome shifts simply as “more good bacteria” can be misleading.

This was a small study and it measured changes in microbial composition rather than proving a clinical health outcome.

But it shows something important:

Relatively small amounts of a fermentable fibre can measurably change the microbial ecosystem.

And the changes are not always as predictable as “this ingredient increases this one bacterium”.

Can prebiotics influence things outside digestion?

Researchers are now asking exactly that question.

In a 2024 Nature Communications study, 36 pairs of twins aged 60 and over took part in a double-blind randomised trial. Within each twin pair, one received a daily inulin/FOS prebiotic and the other received placebo for 12 weeks. Everyone also received resistance exercise guidance and branched-chain amino acids.

The prebiotic changed the microbiome, particularly increasing Bifidobacterium. It did not improve the study's primary physical outcome, but the prebiotic group performed significantly better on a combined cognitive score and made fewer errors on a paired-associates memory task.

This does not establish that prebiotics generally improve cognition. It was a relatively small study in older adults, and cognition was not the primary outcome.

But it is part of the reason microbiome research has moved far beyond bowel movements.

Researchers increasingly want to understand what happens after microbes metabolise what we feed them, and whether those products can influence systems beyond the intestine.

There is another difference between probiotics and prebiotics before they even reach you

Probiotics have one unavoidable manufacturing challenge.

They are alive.

For something to be a probiotic, the relevant microorganisms need to remain viable in adequate numbers.

But microorganisms can lose viability during manufacturing and storage. Temperature, oxygen, moisture, drying conditions and the surrounding food or supplement matrix can all matter. They then face another set of challenges after swallowing, including gastric conditions, digestive enzymes and bile.

This is why probiotic formulation can involve freeze-drying, protective coatings, microencapsulation and carefully controlled packaging and storage.

A label containing “10 billion CFU” is only useful if the intended organisms remain viable at the relevant point in the product's life.

Prebiotics don't need to stay alive

That gives non-living prebiotic ingredients a practical advantage.

They do not have a bacterial viability problem because there is nothing that has to remain alive during manufacturing and storage.

That does not mean every prebiotic is automatically stable. Carbohydrates can still be affected by processing, heat, acidity and moisture depending on their chemistry.

But the technical problem is different.

You are trying to preserve the structure and function of an ingredient rather than keep billions of living organisms alive.

PHGG is an interesting example. Published physicochemical research describes it as highly soluble, low in viscosity and resistant to conditions including heat and acidity, properties that have made it useful in food formulation. More recent experimental work has also reported higher thermal stability in partially hydrolysed guar material compared with untreated guar gum.

So when comparing probiotics and prebiotics, manufacturing is not a boring detail.

It changes the problem you are trying to solve.

Does that mean prebiotics are better than probiotics?

No.

They are different tools.

There is evidence for particular probiotic strains in particular circumstances, and the word strain matters. “Probiotics” are not one uniform treatment any more than “medicines” are one medicine.

Equally, prebiotics do not work identically in everyone, and adding more fermentable fibre is not automatically better. Dose, type of fibre, starting diet, symptoms and the resident microbiome can all affect the response.

The more interesting distinction is this:

A probiotic asks whether introducing particular microorganisms can create a useful effect.

A prebiotic asks whether we can alter the activity of microorganisms already living there.

Recent research suggests that second question is much bigger than simply “feeding good bacteria”.

It is about microbial competition, cross-feeding, fermentation, metabolites and the fact that two people can eat exactly the same substrate and their microbial ecosystems may do different things with it.

Maybe we have been thinking about the gut in the wrong way

For years, gut health was often communicated like gardening.

Add good bacteria.

Remove bad bacteria.

Increase diversity.

But the microbiome does not appear to work in such neat categories.

An adult gut is already a densely occupied ecosystem. New bacteria do not necessarily establish themselves there. Existing microorganisms compete and cooperate. One species can feed another. The same fibre can produce different responses depending on who is already present.

That changes the question.

Instead of simply asking:

Which bacteria should I add?

Microbiome science is increasingly asking:

What are the bacteria already there capable of doing, and what happens when we feed them differently?

That may turn out to be the much more interesting part.

References
Gibson GR, Hutkins R, Sanders ME, et al. Expert consensus document: The International Scientific Association for Probiotics and Prebiotics consensus statement on the definition and scope of prebiotics. Nature Reviews Gastroenterology & Hepatology. 2017.
Hill C, Guarner F, Reid G, et al. The International Scientific Association for Probiotics and Prebiotics consensus statement on the scope and appropriate use of the term probiotic. Nature Reviews Gastroenterology & Hepatology. 2014.
Sinha AK, Laursen MF, Brinck JE, et al. Dietary fibre directs microbial tryptophan metabolism via metabolic interactions in the gut microbiota. Nature Microbiology. 2024.
Mukhopadhya I, Louis P. Gut microbiota-derived short-chain fatty acids and their role in human health and disease. Nature Reviews Microbiology. 2025.
Song D, Feng G, Ma Y, et al. Gut microbiome predicts personalized responses to dietary fiber in prediabetes: a randomized, open-label trial. Nature Communications. 2025.
Baseline intestinal microbiota composition influences response to a real-world dietary fiber intervention. npj Biofilms and Microbiomes. 2025.
Zmora N, Zilberman-Schapira G, Suez J, et al. Personalized Gut Mucosal Colonization Resistance to Empiric Probiotics Is Associated with Unique Host and Microbiome Features. Cell. 2018.
Edelman M, Wang Q, Ahnen RT, Slavin J. The Dose Response Effects of Partially Hydrolyzed Guar Gum on Gut Microbiome of Healthy Adults. Applied Microbiology. 2024.
Ni Lochlainn M, Bowyer RCE, Moll JM, et al. Effect of gut microbiome modulation on muscle function and cognition: the PROMOTe randomised controlled trial. Nature Communications. 2024.
Delzenne NM, Bindels LB, Neyrinck AM, et al. The gut microbiome and dietary fibres: implications in obesity, cardiometabolic diseases and cancer. Nature Reviews Microbiology. 2025.
Bacterial viability retention in probiotic foods: a review. Critical Reviews in Food Science and Nutrition. 2025.

This journal is for general education and is not medical advice.

Why can ADHD feel manageable one week and much harder the next? Why does focus sometimes disappear just before your period? And why do some women say their ADHD medication feels less effective at exactly the same point every month?

For a long time, most ADHD research barely considered the menstrual cycle.

That is beginning to change.

In a 2026 study of 600 women who reported an ADHD diagnosis and ADHD symptoms, 88.6% of premenopausal participants who were not using hormonal therapy said their ADHD symptoms changed across their menstrual cycle, with worsening most commonly reported during the luteal phase.

Another 2026 study followed 30 women with ADHD taking amphetamine-based medication every day for 35 days. Their ADHD symptoms changed significantly across the cycle and were most severe during menstruation and mildest during the mid-follicular phase.

So yes: for some women, ADHD really can feel different depending on where they are in their cycle.

The interesting question is why.

First, what actually happens to your hormones across the menstrual cycle?

The menstrual cycle is not simply “high hormones” followed by “low hormones”. Oestrogen and progesterone rise and fall in different patterns across the month.

At the beginning of the cycle, during menstruation, both oestrogen and progesterone are relatively low. During the follicular phase, oestrogen begins to rise and usually reaches a peak around ovulation. It then drops after ovulation.

During the luteal phase, progesterone rises significantly and oestrogen has a smaller second rise. If pregnancy does not occur, both hormones fall again in the days before menstruation.

Those changes matter because ovarian hormones do not only act on the reproductive system.

They act on the brain too.

What does oestrogen have to do with ADHD?

ADHD involves several brain systems, including dopamine and noradrenaline pathways involved in attention, motivation, reward, working memory and behavioural control.

Oestrogen, particularly estradiol, is also active in the brain.

Research shows that estradiol can influence dopamine production, release, receptors, transport and breakdown. It also interacts with serotonin and glutamate systems. Many of these findings come from laboratory and animal research, so the exact effects in the human ADHD brain are still being worked out, but the biological overlap is significant enough that researchers are now studying it directly.

A 2025 systematic review of ADHD and sex hormones concluded that lower-oestrogen environments were the hormonal state most consistently associated with worsening ADHD symptoms, although it also stressed that the evidence base remains small. Only 11 studies met the criteria for the entire review.

So the developing theory is not simply that “low oestrogen causes ADHD”.

ADHD is a neurodevelopmental condition that exists throughout the cycle.

The idea is that changing hormone levels may change the environment in which an ADHD brain is operating.

What happens after ovulation?

This is one of the most interesting parts of the research.

A study followed 32 young women across 35 days, measuring their hormones through daily saliva samples and recording ADHD symptoms each evening.

When estradiol decreased in combination with higher progesterone or testosterone, ADHD symptoms tended to be higher the following day, particularly in women with higher baseline impulsivity.

Later analysis of this work reported that declines in estradiol predicted clinically meaningful increases of around twofold in inattention and hyperactivity-impulsivity symptoms in some participants.

There is an important detail here: these were community-recruited young women rather than a sample made up entirely of women diagnosed with ADHD.

So the finding is fascinating, but it should not be read as “everyone with ADHD gets twice as symptomatic before their period”.

What it does show is that changes in ovarian hormones can be associated with surprisingly large changes in ADHD-like symptoms within the same person.

And the pattern may not be identical for every ADHD symptom

Researchers have proposed that different points of the cycle may affect different symptoms.

The drop in oestrogen immediately after ovulation has been associated with increases in hyperactivity and impulsivity, particularly in more impulsive individuals.

Later in the cycle, as oestrogen falls again before menstruation, inattention and executive-function difficulties may become more noticeable.

A newer 2026 study in women actually being treated for ADHD adds weight to this idea. Researchers asked 30 women taking amphetamine salts to record their symptoms and mood daily through an entire menstrual cycle. Symptoms were highest during menstruation, when oestrogen and progesterone are low, and comparatively lower during the mid-follicular phase as oestrogen rises.

The changes in ADHD symptoms also tracked changes in negative mood.

Which helps explain why the experience can feel like more than simply “I can't concentrate”.

For some women, the difficult part of the cycle may involve attention, motivation, irritability, emotional regulation and the ability to get started all changing at the same time.

Why can ADHD medication feel less effective before your period?

This is reported frequently enough that researchers have started investigating it, but the evidence is still early.

Stimulant ADHD medications work partly by increasing signalling through dopamine and noradrenaline systems. If ovarian hormones are also influencing those same neurotransmitter systems, it is biologically plausible that medication response could feel different at different points of the cycle.

There are some older clues. Studies in healthy women found that the subjective effects of amphetamine were stronger during the follicular phase than the luteal phase, and in one study greater effects were associated with higher oestrogen levels. These were not ADHD treatment studies, so they cannot tell us how prescribed medication should be managed, but they helped establish that ovarian hormones can alter responses to stimulants.

More recently, clinicians reported a small case series involving nine women with ADHD who consistently experienced worsening ADHD and mood symptoms in the premenstrual week, alongside the feeling that their usual medication was no longer working as well.

Their clinicians temporarily increased stimulant medication during the premenstrual period. All nine reported improvements in areas including inattention, irritability and energy, with minimal adverse effects reported during follow-up.

But nine people is a very small study. There was no placebo group and the researchers themselves describe the findings as preliminary.

So this is not evidence to change medication doses yourself.

It is evidence that a question women have been asking for years is finally being studied.

A 2025 review of the available literature reached a similar conclusion: several studies suggest ADHD symptoms worsen during the luteal phase, but cycle-dependent stimulant efficacy remains seriously under-researched.

What does progesterone have to do with it?

Oestrogen tends to get most of the attention, but progesterone may matter too.

After ovulation, progesterone rises substantially. One of its metabolites is allopregnanolone, a neuroactive steroid that interacts with GABA-A receptors, part of the brain's major inhibitory signalling system.

This becomes particularly interesting when we look at PMDD.

Research increasingly suggests that PMDD is not simply caused by having “too much” or “too little” of a particular hormone. Instead, people with PMDD appear to have an altered brain response to otherwise normal hormonal fluctuations, with allopregnanolone and GABA signalling among the mechanisms being investigated.

And ADHD and PMDD appear to overlap much more often than researchers expected.

ADHD and PMDD: the numbers are striking

A 2025 British study looked at 715 women aged 18 to 34.

Among participants reporting a clinical ADHD diagnosis, 31.4% met screening criteria for provisional PMDD.

Among women without ADHD, the figure was 9.8%.

That is more than three times as high.

Researchers also looked at participants whose ADHD symptoms and impairment met a validated questionnaire threshold, even if they did not report a formal diagnosis. In that group, 41.1% screened positive for provisional PMDD.

Women with high ADHD symptoms alongside depression and/or anxiety had the highest observed risk, at more than four times that of the comparison group.

These are substantial differences, but there is an important word in the study: provisional.

Participants completed screening questionnaires rather than undergoing prospective PMDD diagnosis, and the study was cross-sectional. It can show an association between ADHD and PMDD symptoms, but it cannot tell us that one condition causes the other.

Researchers do not yet know exactly why the overlap exists.

One possibility is that both involve heightened sensitivity to changes in neurotransmitter systems affected by ovarian hormones. Another is that there are overlapping biological, psychological or genetic vulnerabilities.

The answer may involve several of these at once.

So why can the week before your period feel especially difficult?

The late luteal phase creates several changes at the same time.

Oestrogen is falling. Progesterone and allopregnanolone are changing. Premenstrual physical symptoms may appear. Sleep, appetite and mood can change. For someone with ADHD, these changes may be happening on top of existing differences in attention, executive function and emotional regulation.

This may help explain why a task that felt straightforward the previous week can suddenly feel disproportionately difficult.

It is not necessarily that your ADHD has permanently become worse.

The conditions around it have changed.

And this may be why looking only at a monthly or yearly average can miss something important.

The newest research suggests this is very common

The 2026 survey of 600 women is particularly interesting because researchers asked directly about symptoms across hormonal stages.

Among premenopausal participants who were not taking hormone therapy, almost nine in ten reported some change in ADHD symptoms across their menstrual cycle, most commonly worsening during the luteal phase.

That study relied on people remembering and reporting their own symptom patterns, which means it cannot establish the size of the biological effect.

But put it beside daily hormone studies, the 2025 systematic review and the new prospective medication study, and a pattern is beginning to appear.

For at least some women with ADHD, symptoms are not equally intense every day of the month.

What can you actually do with this information?

One of the simplest things may be to look for your own pattern.

If ADHD symptoms, mood, sleep or the perceived effect of medication seem to change around the same point each month, recording them alongside your menstrual cycle for a few cycles can give you something much more useful than trying to remember how you felt three weeks ago.

That does not diagnose PMDD and it does not mean every difficult week is hormonal.

But a repeatable pattern can be useful information to take to a GP, psychiatrist or other relevant healthcare professional, particularly if symptoms become significantly more difficult before menstruation or prescribed medication consistently seems to work differently.

Medication changes should always be discussed with the clinician prescribing them rather than made independently.

What the science actually says

The research is not yet big enough to give every woman with ADHD a menstrual-cycle treatment plan.

The 2025 systematic review found only 11 eligible studies. Several of the most interesting prospective studies contain only 30 or so participants. Medication-adjustment evidence is smaller still.

But the signal is becoming difficult to ignore.

Ovarian hormones interact with neurotransmitter systems involved in attention and executive function. ADHD symptoms have been observed to change across the menstrual cycle. Women with ADHD report particularly frequent worsening in the luteal and menstrual phases. PMDD appears substantially more common among women with ADHD. And researchers are beginning to investigate whether medication response changes alongside those fluctuations.

For decades, women were expected to fit an ADHD model largely built from research in males.

The newer science is asking a different question:

What if ADHD is stable, but the biological environment around it changes across the month?

We do not have the full answer yet.

But we finally have enough evidence to take the question seriously.

References
Osianlis E, Thomas EHX, Li Q, et al. ADHD in females: Survey findings on symptoms across hormonal life stages. Journal of Psychiatric Research. 2026;193:208–215. A survey of 600 female participants reporting ADHD found that 88.6% of premenopausal participants not using hormonal therapy perceived cycle-related changes in ADHD symptoms.
Zaritsky R, Reed SC, Evans SM. Changes in ADHD Symptoms and Mood Across the Menstrual Cycle in Females Treated With Stimulants: A Pilot Study. Journal of Attention Disorders. 2026;30(3):329–341. Thirty participants taking amphetamine salts completed daily symptom tracking across 35 days.
Osianlis E, Thomas EHX, Jenkins LM, Gurvich C. ADHD and Sex Hormones in Females: A Systematic Review. Journal of Attention Disorders. 2025;29(9):706–723. Eleven studies met inclusion criteria, with evidence suggesting links between hormonal changes and ADHD symptom severity.
Eng AG, Nirjar U, Elkins AR, et al. Attention-deficit/hyperactivity disorder and the menstrual cycle: Theory and evidence. Hormones and Behavior. 2024;158:105466.
Roberts B, Eisenlohr-Moul T, Martel MM. Reproductive Steroids and ADHD Symptoms Across the Menstrual Cycle. Psychoneuroendocrinology. 2018;88:105–114. Participants provided daily hormone samples and symptom measures over 35 days.
Bendis PC, Zimmerman S, Onisiforou A, Zanos P, Georgiou P. The impact of estradiol on serotonin, glutamate, and dopamine systems. Frontiers in Neuroscience. 2024;18:1348551.
de Jong M, Wynchank DSMR, van Andel E, Beekman ATF, Kooij JJS. Female-specific pharmacotherapy in ADHD: premenstrual adjustment of psychostimulant dosage. Frontiers in Psychiatry. 2023;14:1306194. Preliminary case series of nine women.
Broughton T, Lambert E, Wertz J, Agnew-Blais J. Increased risk of provisional premenstrual dysphoric disorder (PMDD) among females with attention-deficit hyperactivity disorder (ADHD): cross-sectional survey study. British Journal of Psychiatry. 2025;226(6):410–417.
Hantsoo L, Epperson CN. Towards understanding the biology of premenstrual dysphoric disorder: From genes to GABA. Neuroscience & Biobehavioral Reviews. 2023;149:105168.

This Journal is for general education only and is not medical advice. If cyclical symptoms are significantly affecting daily life, or prescribed ADHD medication appears to work differently across the menstrual cycle, speak with the relevant healthcare professional before making changes to treatment.

Why can two people eat the same fibre and respond differently? Why can one person take a probiotic and notice a change, while another barely does?

One reason may be surprisingly simple:

What is already living in your gut matters.

A major 2026 study involving more than 10,000 people found extensive links between diet and the gut microbiome. Researchers identified dietary associations with 669 of the 724 microbial species they studied, as well as most of the microbial pathways they measured.

The microbiome is not simply something food acts on.

It appears to be part of how we respond to food in the first place.

First: there probably isn't one perfect microbiome

Your gut contains a huge community of bacteria, viruses, fungi and other microorganisms.

But scientists still cannot give us one microbial profile and say:

This is what every healthy gut should look like.

Healthy people can have surprisingly different microbiomes. Diet, age, geography, medication, environment and lifestyle can all influence which microbes are present.

And two people can have different species while their microbiomes still perform some of the same biological functions.

So increasingly, microbiome research is moving away from simply asking:

“Which bacteria do you have?”

towards:

“What are those bacteria actually doing?”

Your gut isn't empty space waiting for new bacteria

This becomes particularly interesting when we talk about probiotics.

The simple idea behind a probiotic is appealing: take beneficial bacteria and add them to the gut.

But the gut is already a densely populated ecosystem.

In a detailed human study, researchers gave people an 11-strain probiotic and directly sampled different areas of their gastrointestinal tract.

What happened varied considerably.

Some people's guts allowed more probiotic colonisation. Others resisted it.

And part of that difference was associated with the person's existing microbiome and gut environment.

Another surprising finding was that detecting probiotic organisms in stool did not necessarily mean those bacteria had successfully established themselves on the intestinal lining.

So:

Taking bacteria is not necessarily the same thing as adding permanent new residents.

That does not mean probiotics cannot work. Some may have useful effects while passing through the gut without permanently colonising it.

But it tells us something important about the ecosystem already there.

It has influence.

This is why prebiotics are so interesting

Prebiotics take a different approach.

Instead of introducing live microorganisms, they provide a substance that particular microorganisms already living in the host can use.

This means the same prebiotic fibre can arrive in two different people and meet two very different microbial communities.

And we now have human evidence that this may affect the response.

In 2025, researchers gave 802 people more fibre. The results weren't the same for everyone.

A major study followed 802 adults with prediabetes across eight medical centres.

Participants received either usual care or an additional dietary fibre intervention for six months.

When researchers looked at the whole group, fibre did not significantly improve the study's main metabolic outcomes.

But that was not the end of the story.

When they examined different biological groups, some participants appeared to respond better than others.

Even more interestingly, features of the participants' gut microbiomes before the intervention began helped predict who was more likely to benefit.

Researchers then tested their microbiome-based prediction model using data from two other fibre studies.

We are not yet at the point where a microbiome test can reliably tell everyone which exact fibre to eat.

But the finding supports an increasingly important idea:

The same fibre does not necessarily produce the same response in every gut.

Another 2025 study found the same thing from a different angle

Researchers gave healthy adults either inulin, resistant-starch-rich unripe banana flour or a control.

Before the intervention, participants already had different starting microbiomes. Some were richer in Prevotella, while others were richer in Bacteroides.

Then they received the same dietary interventions.

Their microbial responses were different.

The resistant-starch intervention caused larger microbiome changes in one of the starting groups than the other.

It was a relatively small study, so we should not reduce everyone into simple “microbiome types”.

But again, the message was similar:

What you eat matters. What it arrives into matters too.

Your bacteria also feed each other

The phrase “feed your good bacteria” makes the microbiome sound much simpler than it is.

Microbes do not work alone.

One species can break down a fibre and produce something another species can use. That second organism may then produce another metabolite.

This is known as cross-feeding.

So eating a fermentable fibre can change more than the amount of one particular bacterium. It can change which microbes have access to nutrients, how they compete and what the wider community produces.

Some of the best studied products of microbial fermentation are short-chain fatty acids, including acetate, propionate and butyrate.

Butyrate can be used as an important energy source by cells lining the colon. Other short-chain fatty acids interact with metabolic, immune and intestinal pathways.

Which means something quite interesting happens when you eat certain fibres:

You eat the substrate. Your microbes metabolise it. Your body then interacts with what they produce.

Diet can shape the microbiome more than you might expect

The 2026 study involving 10,068 people gives us one of the clearest recent pictures of how closely diet and the microbiome are connected.

Researchers found associations between specific foods and specific microbes.

Coffee consumption, for example, was strongly associated with Lawsonibacter asaccharolyticus. Yoghurt was linked with Streptococcus thermophilus, while milk consumption was associated with several Bifidobacterium species.

But the broader dietary pattern mattered too.

Minimally processed, nutrient-rich diets were associated with greater microbial diversity, while higher intake of ultra-processed foods was associated with lower diversity.

Many of the relationships between diet and the microbiome also appeared to persist over time.

This was primarily observational research, so it cannot prove that eating one particular food will produce one particular bacterium.

But across more than 10,000 people, what someone ate carried a remarkable amount of information about what researchers found in their gut.

Does this mean a microbiome test can tell you exactly what to eat?

Not yet.

Consumer microbiome testing has moved faster than the science in some areas.

Researchers are increasingly able to identify associations between foods, microbes and individual responses. Machine-learning models are also being developed to predict how different people may respond to particular dietary interventions.

But prediction is not certainty.

We do not currently have a validated system where one stool sample can reliably tell every healthy person which bacteria they are “missing”, which foods they should avoid or exactly which supplement will optimise their microbiome.

There is another complication: stool only gives us one view of a much larger gastrointestinal ecosystem.

So microbiome testing is scientifically interesting.

It is not yet a complete instruction manual.

What does your existing microbiome actually mean for you?

It means there probably isn't one probiotic, fibre or food that creates exactly the same microbial response in everybody.

It means your existing microorganisms can influence whether new bacteria establish themselves.

It means the bacteria already living in your gut can influence how particular fibres are fermented.

And it means the same food can meet two different microbial ecosystems and potentially produce two different biological responses.

For years, gut health was often reduced to:

add good bacteria, remove bad bacteria.

The newer science is much more interesting.

The microbiome behaves like an ecosystem. Its organisms compete, cooperate, feed one another and respond to what arrives.

Which changes the question from:

“What should I add to my gut?”

to:

“What is already there, and what happens when I feed it differently?”

Your starting point matters.

References
Segev T, Barak D, Zahavi L, et al. Diet–microbiome associations in 10,068 individuals from the Human Phenotype Project to guide personalized nutrition. Nature Medicine. 2026.
Joos R, Boucher K, Lavelle A, et al. Examining the healthy human microbiome concept. Nature Reviews Microbiology. 2025.
Song D, Feng G, Ma Y, et al. Gut microbiome predicts personalized responses to dietary fiber in prediabetes: a randomized, open-label trial. Nature Communications. 2025.
Sardá FAH, Giuntini EB, Oliveira A, et al. Baseline intestinal microbiota composition influences response to a real-world dietary fiber intervention. npj Biofilms and Microbiomes. 2025.
Zmora N, Zilberman-Schapira G, Suez J, et al. Personalized Gut Mucosal Colonization Resistance to Empiric Probiotics Is Associated with Unique Host and Microbiome Features. Cell. 2018.
Kujawska M, Hall LJ. Microbe-Diet Interactions and Personalized Nutrition. Annual Review of Food Science and Technology. 2026.

This Journal is for general education and is not medical advice.