Every morning, without fail, Dexter knows. It doesn't matter if it's January with ice on the path or August with the sun already up at five. Before I've finished lacing my boots, he's at the door. We get about two minutes into the woods before he squats, does what he needs to do, and trots back looking enormously pleased with himself.
I've been watching this happen for years now, and I've started thinking of it as a small daily clinical lesson. Because here's the thing — that clockwork reliability Dexter has? Most of the people who come to me with gut problems haven't experienced it in years. Some have never known it at all. They're constipated for three days, then urgently loose. They go when they can, not when their body wants to. The whole process feels unpredictable, anxious, and exhausting.
There is nothing wrong with Dexter's gut specifically. He has a working circadian rhythm, a reliable morning cortisol rise, and a lifestyle that doesn't involve eighteen hours of screen exposure and a 1am bedtime. He doesn't eat at random times. He walks at the same time every day. He sleeps when it's dark. His body knows exactly what time it is — and so does his colon.
That's the whole post right there. But let's go deeper, because the clinical mechanisms are genuinely interesting.
Your Gut Has Its Own Clock — And It's Not Joking
Most people understand the circadian rhythm as a sleep-wake cycle. But what the research has made increasingly clear over the last two decades is that every organ in the body has its own peripheral clock, governed by a set of genes (CLOCK, BMAL1, PER1/2/3, CRY1/2 among others) that respond to light, feeding cues, and temperature.
Your gut is no exception. The gastrointestinal tract has one of the highest densities of circadian clock gene expression in the entire body. Gastric acid secretion, bile production, intestinal motility, mucus secretion, tight junction protein expression — all of these follow a timed biological programme. The gut is not a passive tube waiting for food to fall through it. It has a schedule. It is expecting things to happen at certain times, and when they do, it performs. When they don't, it gets confused.
Clock Genes & GI Function — What the Research Shows
Studies in both human and animal models have demonstrated that disruption of core clock genes (particularly BMAL1 and CLOCK) significantly impairs gut motility, increases intestinal permeability, and alters the composition of the gut microbiome. Shift workers — the classic circadian disruption model — show substantially higher rates of IBS, constipation, and inflammatory bowel conditions than the general population.
A 2019 paper in Nature Reviews Gastroenterology & Hepatology by Summa & Bhanu specifically mapped how misalignment between central (brain) and peripheral (gut) circadian clocks drives GI dysfunction independent of diet — a finding that has significant clinical implications for anyone treating gut health without addressing lifestyle timing.
The MMC: Nature's Housekeeping Crew
Between meals, your small intestine runs a cleaning cycle called the Migrating Motor Complex (MMC). It's essentially a wave of muscular contractions that sweeps the length of the small intestine roughly every 90–120 minutes, pushing debris, bacteria, and undigested material towards the large bowel. Think of it as the janitor that comes in after the restaurant closes and sweeps the floor before morning service.
The MMC is one of the most important and most overlooked mechanisms in gut health. It's the reason you should not be grazing constantly. Every time you eat — even a small snack — you interrupt the MMC cycle. It has to reset from the beginning. For people who snack all day without a three-to-four hour gap between meals, the MMC barely gets off the ground.
"SIBO — small intestinal bacterial overgrowth — is frequently not a problem of too many bacteria but of too little sweeping. A dysfunctional MMC lets residents move into rooms they shouldn't be in."
The MMC is governed partly by the hormone motilin, which peaks in a fasted state and is sensitive to — you guessed it — circadian timing. Research suggests MMC activity is strongest in the early morning hours, which is precisely when a healthy bowel movement should occur. This is not a coincidence. It's the biological equivalent of Dexter at the door at 7am.
The Cortisol Awakening Response: Your Daily Biological Alarm
Within 30 to 45 minutes of waking, a healthy HPA axis produces a sharp spike in cortisol known as the Cortisol Awakening Response (CAR). This is normal and necessary — cortisol is not your enemy in the morning. It mobilises glucose, increases alertness, and — critically for this discussion — stimulates colonic motility through direct action on smooth muscle and indirect stimulation of the enteric nervous system.
The CAR is one of the reasons why healthy people tend to have their first bowel movement in the morning. Cortisol gets the colon moving. It's part of the same biological "rise and prepare" signal that gets your heart rate up, your muscles ready, and your mind alert.
In people with HPA axis dysregulation — which includes a large proportion of my clients, particularly those with chronic stress, long-haul fatigue, or a history of overtraining — the CAR is blunted. The morning cortisol spike is flat, delayed, or absent. And without that signal, the colon doesn't get its morning instruction.
If you're someone who doesn't have a reliable morning bowel movement, the first question I'd ask is: what does your morning cortisol look like? This is one of the things the DUTCH hormone test picks up, and it's often revelatory for clients who've been treating constipation with magnesium and fibre without any real improvement.
Parasympathetic Dominance: You Cannot Eliminate Under Threat
Bowel function requires parasympathetic nervous system dominance. Full stop. The enteric nervous system — the gut's own internal network, sometimes called the "second brain" — receives direct input from the vagus nerve, which carries parasympathetic signals from the brainstem. When the vagus nerve is well-toned and the nervous system is in a state of relative safety and calm, gut motility is smooth, rhythmic, and reliable.
When you are in sympathetic overdrive — stressed, anxious, rushing, over-caffeinated, sleep-deprived — the body has pulled blood supply away from the gut, slowed motility in the small intestine, and prepared you to either fight or run. Digestion and elimination are expensive biological processes. The body will not invest in them when it thinks you're being chased.
The modern human does a remarkable impression of an animal being chased, essentially all the time. Deadlines, notifications, financial worry, relationship friction, unresolved inflammation — all of these activate the same threat response. The gut pays the price.
Vagal Tone and Gut Motility
Low vagal tone — measurable through heart rate variability (HRV) — is consistently associated with slower gut transit time, higher rates of functional constipation, and increased gut permeability. A 2020 review in Frontiers in Neuroscience found that vagus nerve stimulation (both pharmacological and non-pharmacological) significantly improved gut motility outcomes in patients with chronic functional constipation, confirming the direct pathway between nervous system state and bowel function.
Non-pharmacological vagal tone interventions with evidence behind them include: slow diaphragmatic breathing, cold water facial immersion, humming and singing, regular aerobic exercise, and social connection. None of these are available as a supplement. All of them matter.
Why Dexter Gets It Right (And How We Lost It)
Dexter's morning constitutional works because every element of his biology is entrained. He wakes at the same time. He gets direct morning light the moment we step outside. He moves. He eats at predictable times. He doesn't eat after 7pm. He sleeps in darkness. He has no work stress, no financial anxiety, and no Instagram feed.
I'm not suggesting you live like a Labrador (though there are worse aspirations). But the principles are sound:
The gut — your gut — is expecting a regular morning signal. Light → movement → morning cortisol → parasympathetic window → bowel movement. That sequence is written into your biology. It's not optional plumbing. It's a finely timed orchestration, and when you disrupt the timing consistently enough for long enough, the whole performance falls apart.
Add in years of stress, disrupted sleep, irregular eating, and chronic gut inflammation, and what you're left with is a digestive system that no longer knows what time it is, what it's supposed to be doing, or when. The clock genes are still there. They're just no longer receiving coherent inputs.
Engineering Your Morning Constitutional
This is where it becomes practical. You cannot simply decide to have a regular morning bowel movement and make it happen through willpower. But you can systematically rebuild the biological cues that make it likely. This is what I mean when I talk about entrainment.
The Morning Entrainment Protocol
- Same wake time, every day. Including weekends. Social jet lag — the shift in sleep timing between weekdays and weekends — is enough to disrupt circadian entrainment on its own. Pick a time and hold it.
- Light before anything else. Go outside within 15 minutes of waking. Even two to three minutes of morning light without sunglasses signals to your suprachiasmatic nucleus (your central circadian master clock) that the day has started, triggering the downstream cascade that includes your cortisol awakening response. Dexter and I do this together. He has no idea why it matters. It doesn't bother him.
- Movement before sitting. Even a 10-minute walk. Movement stimulates colonic motility directly through both mechanical and neurological mechanisms. Sitting still after waking does not.
- Warm water, first thing. 300–500ml of warm water on an empty stomach stimulates the gastrocolic reflex — the neurological signal from the stomach to the colon that says "something is coming, prepare to move the previous load on." Coffee has a similar effect (stronger, actually), but warm water alone is often sufficient and kinder to the adrenals if yours are already under strain.
- Don't rush. Give yourself a 20–30 minute unhurried morning window. I know this feels impossible to some people. I know you have children, a commute, a full diary. I'm telling you that if you have chronic constipation or IBS, this is not optional. The body will not eliminate under time pressure. It needs a parasympathetic window. Carve it out.
- Consistent breakfast timing. Eat within the same one-hour window each morning. This entrains the peripheral clock genes in the gut directly. Skipping breakfast entirely is not catastrophic for everyone, but eating it at wildly different times defeats the purpose.
- Three-to-four hour gaps between meals. Protect the MMC. Let the housekeeping crew do their job between meals. Stop grazing.
When This Isn't Enough
For a significant proportion of my clients, the entrainment protocol above makes a meaningful difference on its own. Particularly for people whose constipation is primarily functional — driven by lifestyle, stress, and circadian disruption rather than structural or infectious causes.
But for others — and I want to be honest about this — there is more going on. If you've implemented everything above consistently for four to six weeks and your bowel function remains unpredictable, you need to look deeper. The possibilities include:
SIBO (Small Intestinal Bacterial Overgrowth): Bacterial overgrowth in the small intestine creates methane and hydrogen gas that directly inhibits gut motility. It's one of the most common underlying drivers of chronic constipation I see, and it won't resolve with lifestyle changes alone. It needs testing and a targeted protocol.
H. pylori or other GI infections: H. pylori impairs gastric acid production, which has downstream effects on bile and enzyme release, which affects everything from motility to microbiome composition. Again — needs testing.
Thyroid dysfunction: Hypothyroidism — including subclinical hypothyroidism that doesn't show on a standard TSH test but does appear on a full thyroid panel — is a classic cause of slow gut motility. I've seen clients do everything right on the lifestyle side and still be constipated because their T3 is low and their reverse T3 is dominant. You need the full picture.
HPA axis dysregulation: If your cortisol awakening response is genuinely blunted, you need more than a morning walk to fix it. You need a DUTCH hormone test, you need to understand your full diurnal cortisol pattern, and you need a targeted adrenal support strategy. This is something I assess as part of the full TDG programme.
"The lifestyle work is the foundation. But foundations don't fix structural damage. When the body is not responding to the basics, testing is not optional — it's the only honest next step."
The Takeaway
Dexter does not think about any of this. He wakes up, he goes outside, he goes to the toilet, and he has an excellent time doing it. He has a reliable morning constitutional because his biology is working as designed.
You have the same biology. The same clock genes. The same MMC. The same CAR. The same vagus nerve. The difference is not species. The difference is signal coherence — whether the inputs your body receives are consistent enough for the system to know what time it is and what to do.
Most gut health advice skips straight to fibre, probiotics, and elimination diets. These things matter — in the right context, at the right time. But if your circadian rhythm is disrupted, your cortisol awakening response is flat, and your nervous system is spending most of its time in sympathetic overdrive, no amount of psyllium husk is going to give you a reliable morning constitutional.
Start with the timing. Start with the light and the movement and the unhurried morning. And if that doesn't move the needle after a month of honest effort, come and find me. We'll test, not guess.
Stephen Duncan is an FDN-P and MSc-qualified functional health practitioner based in Edinburgh. This post is for educational purposes only and does not constitute medical advice. If you have persistent gut symptoms, consult a qualified practitioner. Key references: Summa KC & Bhanu B (2019), Nature Reviews Gastroenterology & Hepatology; Scheiermann C et al. (2013), Nature Reviews Immunology (circadian control of immunity and epithelial function); Bharucha AE et al. (2022), Gastroenterology (functional constipation review); Browning KN & Travagli RA (2014), Nature Reviews Neuroscience (central nervous system control of gastrointestinal motility).