The Gut–Brain Axis Explained in Simple English (And Why It Runs Your Sleep, Stress & Focus)

December 24, 2025 Jess Montford

Most people think sleep problems live in the head.

They don’t.

They live in the gut - quietly pulling strings behind the scenes while everyone argues about melatonin gummies and blue-light glasses.

If you struggle to switch off at night, feel anxious for no obvious reason, or wake up tired despite “doing all the right things,” there’s a good chance your gut–brain axis is involved.

This isn’t trendy wellness talk. It’s how your nervous system is wired.

The gut–brain axis is the communication system linking your digestive system and your brain. When it works, sleep feels natural. When it doesn’t, your body stays stuck in low-grade alert mode - and no amount of bedtime routines will fix that.

Let’s break it down properly. Simple English. Zero fluff.

What the Gut–Brain Axis Actually Is

The gut–brain axis is a two-way communication loop between your gut and your central nervous system.

Your brain sends signals down to control digestion, immune response, and gut movement. At the same time, your gut sends signals up that influence mood, stress perception, focus, and - yes - sleep quality.

This communication runs through a few key channels:

The vagus nerve (a physical nerve connecting gut to brain)
Hormones like cortisol and serotonin
Neurotransmitters such as GABA
Immune signals triggered by inflammation
Metabolites produced by your gut bacteria

Think of your gut as mission control. If it’s calm and well-fuelled, your brain relaxes. If it’s inflamed or underfed, your brain assumes something’s wrong - and stays switched on.

A switched-on brain doesn’t sleep well. Period.

Your Gut Makes More “Brain Chemicals” Than Your Brain

Here’s a stat that usually stops people mid-sentence:

Around 90% of serotonin is produced in the gut.

Serotonin isn’t just the “happy” neurotransmitter. It’s also the precursor to melatonin - your sleep hormone.

So when gut health dips, this tends to happen:

Serotonin production drops
Melatonin synthesis suffers
Sleep onset slows
Deep sleep becomes harder to reach

This is why gut issues often show up as poor sleep before digestion feels noticeably bad.

You’re not overthinking things.
Your biochemistry is off.

The Vagus Nerve: Your Built-In Calm Switch

The vagus nerve is the main highway between your gut and your brain. It controls how easily your body can enter the parasympathetic (“rest and digest”) state.

When vagal tone is strong:

Heart rate slows
Breathing deepens
Cortisol drops
Digestion improves
Sleep comes easier

But when the gut is inflamed, bloated, or irritated, vagal signalling weakens. The brain receives fewer “all clear” messages and stays partially alert.

That’s when you get the classic:

Tired but wired feeling
Racing thoughts at night
Shallow, broken sleep

This is why fixing sleep often starts below the neck, not in your head.

Gut Inflammation Quietly Cranks Up Cortisol

Your body treats gut inflammation like a threat.

Ultra-processed food, alcohol, chronic stress, antibiotics, and low fibre intake all irritate the gut lining. When that lining becomes compromised, inflammatory signals leak into circulation.

The result? Elevated cortisol.

Not just during the day - at night, when it’s supposed to drop.

That leads to:

Reduced deep sleep
More night-time awakenings
Poor recovery
Groggy mornings

We break this mechanism down further in The Ultimate Guide to Gut Health and Sleep, where we explain how inflammation directly disrupts sleep architecture and next-day performance.

The Microbiome Isn’t a Trend - It’s a Control System

Your gut bacteria don’t just help digest food. They actively influence your nervous system.

A healthy microbiome helps:

Regulate cortisol
Support serotonin and GABA production
Stabilise blood sugar
Reduce systemic inflammation

A disrupted microbiome does the opposite.

This is where the wellness industry usually messes things up. Everyone shouts “take probiotics,” but forgets that bacteria need the right environment to work.

That environment is built through:

Adequate fibre (prebiotics)
Low inflammation
Stable digestion
Stress regulation

That’s why Moongreens DeepDrift™ takes a systems approach - combining probiotics, prebiotic fibres, magnesium, and adaptogens to support gut signalling and nervous system calm instead of treating them as separate problems.

GABA: Why Your Brain Won’t Shut Up at Night

GABA is your brain’s main inhibitory neurotransmitter. It’s the chemical equivalent of telling your nervous system to calm down.

Here’s the important bit: gut health directly influences GABA production and sensitivity.

When the gut is healthy:

GABA signalling improves
Anxious thought loops reduce
Sleep onset becomes easier
Deep sleep increases

When the gut is inflamed or undernourished, GABA signalling weakens. Your brain stays noisy - even when you’re exhausted.

This is why improving digestion often reduces night-time anxiety without touching sleep meds or stimulants.

Blood Sugar, the Gut, and 3am Wake-Ups

If you regularly wake up between 2–4am, this isn’t random.

Poor gut health affects glucose absorption and insulin sensitivity. When blood sugar drops too low overnight, your body releases cortisol and adrenaline to correct it.

Those hormones wake you up.

Supporting gut health helps stabilise blood sugar, which keeps cortisol low overnight - and lets you stay asleep.

We explore this exact loop in Probiotics for Sleep: Do They Really Work?, where we separate real gut–sleep science from marketing hype.

So What Actually Supports the Gut–Brain Axis?

No extremes. No detox nonsense. Just fundamentals done well.

That means:

Eating enough fibre consistently
Reducing ultra-processed food
Managing stress (yes, really)
Supporting digestion
Layering nutrients that calm the nervous system

This is also why an all-in-one nightly formula like Moongreens DeepDrift™ can be effective - it supports gut health, cortisol regulation, and neurotransmitter balance in one simple ritual instead of asking you to juggle six different supplements.