Stress, sleep and the gut

Healthy state

The autonomic nervous system, which works without our will, has two counterparts. The sympathetic system is the “fight or flight” system: it wakes you up, raises pulse and blood pressure and sends blood to the muscles. The parasympathetic system, which includes the vagus nerve Vagus nerve The tenth cranial nerve and the most important nerve of the rest-and-digest system. It connects the brainstem and the abdominal organs; most of its fibres tell the brain what is happening in the gut. More in the chapter “The vagus nerve: the data line between belly and head”, is the “rest and digest” system. Only in this state do saliva, stomach acid, pancreatic enzymes and bile flow freely, does the gut move in even waves, and does the lining have the strength for absorption and defence. In daily life the two systems keep alternating. Short stress, such as before a presentation, is normal and does no harm.

On the stress side there are two pathways. The fast one runs through nerves and the hormone adrenaline. The slower one is the stress axis: the brain releases a signal hormone called CRH, and the adrenal gland then makes cortisol. Cortisol rises shortly after waking, gets us going and falls again over the day. But CRH does not act only in the brain; it also acts directly in the gut, where nerves, immune cells and mast cells Mast cells Immune cells beneath the skin and mucous membranes, filled with histamine and other messengers. They raise the alarm in allergies, infections and stress. More in the chapter “Mast cells: alarm cells in the gut, histamine and the disputed MCAS” (immune cells that store messengers such as histamine Histamine A messenger substance of the body that is also found in aged and fermented foods. Too much can cause headache, flushing or diarrhoea. More in the chapter “Histamine intolerance: much searched, little proven”) carry receptors for it. That is why the belly often notices stress before the head does, for example as sudden urgency before an exam.

The gut also has its own rhythm. The central body clock in the brain is set mainly by light. In addition there are clocks inside the cells of stomach, gut and liver that follow mealtimes closely. During the day gut movement is livelier, at night calmer. The colon is especially active in the morning, which is why the urge to go often comes after getting up and breakfast: food in the stomach triggers movement in the colon, the so-called gastrocolic reflex. Between meals and at night, a “cleaning wave” runs through the small intestine about every one to two hours, the migrating motor complex Migrating motor complex A wave of muscle contractions that sweeps the stomach and small intestine empty about every one and a half to two hours when fasting. Eating interrupts it. More in the chapter “Intermittent fasting and meal timing: 16:8, 5:2 and Ramadan”, pushing leftovers and bacteria towards the colon. It only starts in the fasting state, that is, when you are not eating all the time.

Sleep is recovery for the gut. In deep sleep the parasympathetic system dominates, the lining keeps renewing itself and the immune system works calmly. Gut bacteria also fluctuate over the day; this is well shown in mice, much less so in humans.

The link runs both ways. Anyone who wakes at night because of abdominal pain or toilet trips sleeps worse, and anyone who sleeps badly is more sensitive the next day. According to a meta-analysis Meta-analysis An analysis that combines the results of several studies. It is only as good as the studies it includes. of 36 studies, sleep problems are common in people with IBS Irritable bowel syndrome Recurring abdominal pain linked to bowel movements, without tests finding another cause. Today it is understood as a disorder of gut–brain interaction. More in the chapter “The gut-brain axis”.

Disrupted state: causes and effects

When the alarm state persists, a lot shifts. In one person the colon speeds up, stool becomes loose, and diarrhoea and urgency follow. In another the pelvic floor tenses, or the bowel movement is suppressed because there is “no time” right now, and constipation develops. In the stomach, pressure, nausea and fullness can increase. Some people unconsciously tense the abdominal wall and diaphragm or breathe shallowly; the belly then feels bloated even though there is no more gas. Which direction you take depends on predisposition, gut flora Microbiome, gut microbiota The community of all microbes in the gut, mainly bacteria, plus viruses, fungi and archaea. Most live in the colon. More in the chapter “Building microbial diversity” and history. The same level of stress can trigger opposite effects in two people.

Stress also acts on the gut wall. In a small study of healthy volunteers, the stress hormone CRH made the small intestine more permeable via mast cells. The connections between the lining cells (tight junctions Tight junctions Protein connections that hold the cells of the gut wall tightly together. They determine what can pass between the cells. More in the chapter “The gut barrier”) then open more easily, more substances from the gut contents reach the immune cells underneath, and this can strengthen irritation and hypersensitivity. This is a mechanism, not a diagnosis of its own: whether increased permeability alone causes symptoms is unclear.

Stress does not only make the gut faster or slower; it also sharpens the brain's attention to signals from the belly. Normal stretching is then more likely to be rated as pain. This is the core of the gut-brain axis, which has its own chapter. On top of that comes anticipatory anxiety: if you fear needing the toilet urgently on the road, you tense up and watch your belly, and that is exactly what triggers the symptoms.

That stress and lack of sleep change the microbiome has mainly been shown in animal studies. In humans the picture is thinner: a review of 13 human studies found associations between psychological stress and the gut flora, but no proof of cause and effect. In a study of healthy people, a few nights of strongly shortened sleep barely changed the composition of gut bacteria. A systematic review of 41 human studies did find associations between sleep duration, sleep disorders or shift work and the gut flora, but with inconsistent results, especially for diversity. A realistic reading: in the short term, stress and lack of sleep act mainly through nerves, hormones and changed eating behaviour. The microbiome probably follows only after a long-lasting state.

Shift work, night-time meals and frequent time shifts pull the clocks apart: the central clock in the brain follows light, the clocks in the gut follow food. Anyone who eats at night sends the gut the signal “day” while brain and hormones say “night”. A recent review describes links between a disturbed day-night rhythm and disturbed gut movement and suspects that women react more sensitively. The evidence comes mostly from animal studies and small studies, though. Eating constantly between meals also suppresses the cleaning wave in the small intestine.

This closes the circle: stress and poor sleep worsen the gut, symptoms disturb sleep, tiredness lowers stress tolerance. There are two misunderstandings. “It’s just stress” is not always true: coeliac disease Coeliac disease An autoimmune disease in which gluten damages the lining of the small intestine. It is diagnosed with a blood test and usually a biopsy while you are still eating gluten. More in the chapter “Lactose, fructose, gluten: making sense of intolerances”, inflammation, thyroid problems or infections must be ruled out, especially with warning signs. And “you’re imagining it” is not true either, because the changes in the gut are measurable. How it works for you is shown by a diary over a few weeks with symptoms, stress and sleep.