【For Those Who Can’t Shake Fatigue and Irritability】Can Fasting Balance Your Autonomic Nervous System? How It Works, the Right Approach, and Precautions

“I’m tired from the moment I wake up.” “I get irritated over the smallest things.” Does that sound like your everyday life?

The truth is, that chronic fatigue and irritability very often come down to more than just stress or lack of sleep; an imbalanced autonomic nervous system is at the root of it.

If you can’t shake your fatigue, it may not be because you lack willpower or grit.

And recent research has been making it clear that fasting is a pretty effective approach for rebalancing the autonomic nervous system.

In this article, I’ll explain in plain terms how fasting works on the autonomic nervous system, along with concrete, practical ways to relieve chronic fatigue and irritability.

The Causes of Chronic Fatigue and Irritability, and How They Relate to the Autonomic Nervous System

What Is the Autonomic Nervous System?

The autonomic nervous system is the part of the nervous system that runs your body without you having to think about it, controlling things like breathing, heart rate, digestion, and body temperature.

There are two main types, and your body’s condition is maintained by the balance between the “sympathetic nervous system,” which handles activity and tension mode, and the “parasympathetic nervous system,” which handles rest and recovery mode.

If you compare it to a car, the sympathetic nervous system is the accelerator, and the parasympathetic nervous system is the brake.

When you’re healthy, the two switch naturally to suit the situation: accelerator during the day, brake at night.

When the sympathetic nervous system is dominant all the time, your body stays stuck in “fight mode.”

It’s like keeping your foot on the gas pedal and being unable to ease off even when it’s time to rest.

As a result, symptoms such as chronic fatigue, irritability, insomnia, and indigestion become more likely.

On the flip side, there is also a type of imbalance where the brake works too hard, leaving you sluggish and unmotivated even during the day.

Common Signs That Your Autonomic Nervous System Is Out of Balance

Signs that commonly appear when your autonomic nervous system is out of balance include the following.

If several of these apply to you, consider the possibility that your body’s accelerator and brake aren’t switching properly.

Also, if dizziness, palpitations, insomnia, or similar symptoms continue for a long time and interfere with your daily life, another condition such as autonomic dysfunction may be hiding behind them, so please consult a medical professional first.

Why So Many People Today Have an Imbalanced Autonomic Nervous System

Modern life is full of things that throw off the autonomic nervous system.

A lifestyle of “overeating and eating constantly” in particular never lets the gut rest, and wears out the brain and autonomic nervous system through the gut-brain axis (the nerves connecting the gut and the brain).

In a great many cases, this becomes a hidden cause of chronic fatigue and irritability.

If you keep snacking from morning to night or eat right before bed, your stomach and intestines end up working overtime even at night.

When you think about this connection, it makes sense that a late dinner can make it hard to fall asleep, or that a heavy stomach in the morning can make waking up feel sluggish, doesn’t it?

The Vicious Cycle of Sleep Deprivation and Overeating

Another point you shouldn’t overlook is that an imbalanced autonomic nervous system affects both sleep and appetite, and they affect each other.

When you don’t get enough sleep, your body tends to ramp up your appetite to make up for the fatigue, and you’re likely to crave sweet or fatty foods.

If you then eat late at night, your digestive system’s overtime makes your sleep shallow, and you end up even more tired the next day.

It’s a vicious cycle, like taking out a new loan to pay off an old one.

The shortcut to breaking this cycle is to set a rule about timing: “when you eat, and when you stop eating”.

How Fasting Rebalances the Autonomic Nervous System

(1) Reducing the Strain on Digestion Helps the Parasympathetic Nervous System Recover

Digestion uses a surprisingly large amount of energy.

Every time you eat, the sympathetic nervous system is activated, and your body does the “work” of digestion.

During a fast, there’s no digestive work to do, so the parasympathetic nervous system more easily takes the lead, promoting deep recovery in the body.

The feeling of “my body feels lighter” or “my head feels clear” after fasting is linked to this activation of the parasympathetic nervous system.

In particular, if you spend the few hours before bed on an empty stomach, your digestive system doesn’t have to work overtime while you sleep, so your body can focus on “resting.”

(2) Stable Blood Sugar Reduces the Stress Response

When your blood sugar keeps shooting up and crashing down, your body releases adrenaline and cortisol to try to bring it back.

This hormonal response stimulates the sympathetic nervous system and becomes a direct cause of irritability, anxiety, and poor concentration.

In other words, the roller-coaster rise and fall of your blood sugar is what creates your mood swings.

When fasting reduces how often you eat, those blood sugar swings are kept in check, and the release of stress hormones stabilizes.

This leads to less irritability.

(3) Improving the Gut Environment Rebalances the Gut-Brain Axis

The gut and the brain are connected by the vagus nerve, and improvements in the gut environment directly affect the brain and the autonomic nervous system.

Giving your gut a rest during a fast promotes autophagy (the cells’ self-repair process), which leads to repair of the intestinal lining and improvements in the gut microbiome.

Autophagy is a process in which cells break down and recycle their own worn-out parts, a kind of “deep cleaning for your cells.”

Since about 90% of serotonin, known as the happiness hormone, is made in the gut, a gut in good shape is also thought to influence a stable mood through the gut-brain axis.

(4) Reducing Inflammation Calms an Overactive Nervous System

Chronic inflammation makes the nervous system hypersensitive, creating a state where you overreact to even minor stimuli.

Chronic inflammation is like a small fire that keeps smoldering inside your body.

The anti-inflammatory effect of fasting calms this overactive nervous system and gradually turns your body into one that doesn’t tire as easily.

(5) A Regular Meal Rhythm Sets Your Body Clock

Inside your body is a “body clock” that keeps a rhythm on a roughly 24-hour cycle.

It has become clear that the body clock is influenced not only by light but also by the timing of your meals.

When you eat during the same window every day and stop eating at the same time, your body finds it easier to tell “now is the time to be active” from “now is the time to rest.”

You could say this rhythm is part of what makes the switch between the accelerator and the brake run smoothly.

Practical Ways to Fast to Rebalance Your Autonomic Nervous System

Starting With 16-Hour Fasting (16:8) Is Ideal

When fasting to improve your autonomic nervous system, 16-hour fasting (the 16:8 protocol) is the easiest to adopt and the most effective.

Stop eating after 8 p.m. and fast until noon the next day; this alone lets you make the most of the time your parasympathetic nervous system has to recover during sleep.

16:8 means going 16 hours without eating and having your meals within the remaining 8 hours.

If hunger is hard for you, it’s easier to stick with if you shorten the fast to 12 to 14 hours for just the first week and then gradually work up to 16 hours.

Sample Recommended Schedule

What matters isn’t only the length of time, but also eating during roughly the same window every day and stopping at roughly the same time.

What to Avoid During a Fast to Keep Your Autonomic Nervous System Balanced

Recommended Drinks During a Fast

It’s a good idea to choose what you drink during a fast with its effect on your autonomic nervous system in mind.

All of these have almost no calories, so they ease hunger without interfering with the benefits of fasting.

Caffeinated drinks like coffee and green tea can stimulate the sympathetic nervous system if you have too much, so keep an eye on how much you drink.

How to Spend Your Time During a Fast

During a fast, drink water or tea regularly and don’t ignore your thirst.

Also be mindful of creating an environment where the parasympathetic nervous system can do its job, such as dimming the lights a little at night and putting your phone down early.

5 Ways to Support Your Autonomic Nervous System Through Food

(1) Boost Serotonin With Foods Containing Tryptophan

Tryptophan, the building block of serotonin, is abundant in eggs, bananas, nuts, dairy products, and soy products.

Consciously including these in your first meal after a fast helps keep your mood stable through the afternoon and beyond.

If you’re doing 16:8 and skipping breakfast, you can work them in just by adding an egg dish, natto (fermented soybeans), yogurt, or the like to your first meal at lunchtime.

(2) Ease Nervous Tension With Magnesium

Magnesium is sometimes called a “natural relaxant” because it calms nerve excitation.

It’s plentiful in foods like spinach, nuts, tofu, and dark chocolate.

People today tend to fall short on magnesium, so make a conscious effort to get enough.

(3) Calm Inflammation and Stabilize Your Nerves With Omega-3 Fatty Acids

The omega-3s in oily fish (mackerel and sardines), walnuts, and flaxseed oil help calm nerve inflammation and stabilize the autonomic nervous system.

Try to aim for oily fish at least three times a week.

When cooking feels like too much trouble, canned mackerel or canned sardines make it easy to keep it up.

(4) Rebalance Your Autonomic Nervous System From the Gut With Fermented Foods

Fermented foods such as yogurt, miso (fermented soybean paste), natto (fermented soybeans), and kimchi improve the gut microbiome and help with serotonin production.

If you make a habit of including some fermented food at every meal, you can keep your gut-brain axis in balance on an ongoing basis.

Small touches like adding a bowl of miso soup or a pack of natto are enough to start with.

(5) Cut Back on Sugar and Processed Foods to Prevent Blood Sugar Swings

I’ve already explained how blood sugar ups and downs trigger stress hormones.

Just cutting back on white carbohydrates, sweets, and juice, and swapping them for fiber-rich foods (vegetables, beans, and whole grains), will start to stabilize your blood sugar.

Eating in the order “vegetables → protein → carbohydrates” is another easy way to prevent sharp blood sugar spikes.

A Sample Day of Meals Using the 5 Points

Here’s a sample day of meals that follows the recommended schedule and incorporates the five points.

There’s no need to stock up on special ingredients; just adding things little by little to your usual meals is enough.

Start with whatever form you can keep up without strain.

Can Fasting Throw Off Your Autonomic Nervous System? Precautions and Who Should Avoid It

Pushing Through Hunger Too Hard Revs Up the Sympathetic Nervous System

If you fast the wrong way, rather than rebalancing your autonomic nervous system, it can actually throw it off.

When you keep enduring intense hunger, your body perceives it as stress and switches on the sympathetic nervous system, ramping up activity mode.

As a result, you may experience problems such as irritability, shaky hands, palpitations, and trouble falling asleep.

Rather than forcing yourself to endure more and more, it’s important to adjust things so you can keep going comfortably.

Watch Out for Binging After a Fast and Getting Your Recovery Meal Wrong

After a fast, your stomach and intestines are still in rest mode, so suddenly eating a large amount can cause stomach pain, sluggishness, and lightheadedness.

Also, wolfing down sweet or fatty foods right after a fast can send your blood sugar swinging wildly, which may become the trigger that knocks your autonomic nervous system out of balance.

When you break your fast, eat soups, vegetables, protein, and the like slowly, chewing them well.

On Days You Feel Unwell, Don’t Hesitate to Take a Break

On days when you’re short on sleep, feel like you’re coming down with a cold, or are under heavy stress, your body already has its foot on the accelerator.

Pushing yourself to fast on days like that can put even more strain on your body.

When you’re not feeling well, take a break from fasting and prioritize taking care of your body with warm soup and easy-to-digest meals.

Even if you take a day off, it’s no problem as long as you get back to your usual rhythm the next day.

Stop If You Notice These Signs

If these symptoms appear, don’t push yourself; stop fasting, have some fluids and a light meal, and rest.

If the symptoms continue, see a doctor promptly.

People Who Should Avoid Fasting on Their Own Judgment

For people like the following, fasting can put a strain on the body.

If you have a chronic condition or take medication, don’t forget to always talk to your doctor before you start fasting.

If You’ve Been Diagnosed With Autonomic Dysfunction, Talk to Your Doctor

If you’ve already been diagnosed with autonomic dysfunction and are receiving treatment, talk to your doctor before you start fasting.

Fasting is only one way to regulate the rhythm of your daily life; it is not a substitute for medical treatment.

Once your doctor gives you the go-ahead, start with short fasts and keep a record of how you feel in a journal or the like for peace of mind.

Having a record makes it easier to discuss things concretely with your doctor at your next appointment.

Frequently Asked Questions About Fasting and the Autonomic Nervous System

Rebalance Your Autonomic Nervous System With Fasting and Get Back a Body That Doesn’t Tire Easily

Here’s a summary of the key points for relieving chronic fatigue and irritability.

You don’t have to change everything at once.

Start by trying just two things: “no eating after 8 p.m.” and “start the morning with warm water.”

Many people find that after two weeks, they can feel a difference in how they wake up and in their energy during the day.

Even if some days don’t go well, it’s fine as long as you return to your usual times the next day, so keep going without stressing about it.

The quickest route to a balanced autonomic nervous system may not be doing anything special, but rather gradually putting your daily rhythm in order.

▼ Watch the video version of this article