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The 12 Hour Fast: Benefits and How To

By Liam McAuliffe Updated on

A 12-hour fast, AKA 12:12 intermittent fasting, is a method that involves abstaining from food for at least 12 hours, typically overnight. 

Intentionally depriving your body of calories for a specific period of time activates numerous biological processes that offer powerful health benefits, including  

  • Reduced inflammation and decreases in proinflammatory cytokines and immune cells 
  • improve circulating glucose 
  • Improve blood lipid (cholesterol/triglyceride) levels 
  • reduce blood pressure. 
  • regulate blood sugar
  • stimulate human growth hormones
  • activate cellular renewal
  • Promote mental clarity and cognition 
  • Protect against age-related cognitive decline 

Most of these call for fasting periods beyond 12 hours. However, the 12-hour fast can be an easily adopted gateway into longer, more effective intermittent fasting approaches. 

In this article, we’ll look at how to do a 12-hour fast, what to eat, and what benefits you can expect. 

Table of Contents

What is the 12-hour Fast? 

A twelve-hour fast is one of the easiest intermittent fasting approaches to get into. 

Though 12:12 appears mild, when coming from a Standard American Diet, the positive results can be significant and felt within a couple of days. 

Most people are constantly eating and often consume food from the moment before going to bed to the moment they wake up. Most of this “food” is loaded with sugars, grains, and seed “vegetable” oils–all of which are among the most toxic, pesticide-ridden, and inflammatory foods on earth. 

The most effective approach to the 12-hour fast


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Align Your 12-hour fast with Your Circadian Rhythms

Linking your fasting period with circadian rhythms takes advantage of innate metabolic processes in your body that are governed by natural light and dark cycles.

Your circadian rhythm is a 24-hour internal biological clock. 

It regulates many bodily functions, including sleep, hormone secretion metabolism, digestion, immune function, and cognition.

Nearly every cell in your body and 80% of your genes function relative to this light-dark cycle.

When you align your 12-hour fast with the natural light and dark cycle you take advantage of innate internal processes governed by your biological clock.

Studies show that eating during daylight and abstaining from food after sundown results in benefits, including

  • Lower body fat percentage
  • Better sleep
  • Improve digestive symptoms, including abdominal pain, bloating, diarrhea, and constipation.
  • Reduced risk of intestinal dysbiosis (overgrowth of harmful bacteria)
  • Improved recovery of immune cells

This means that if you finish your last meal at sundown, your next meal wouldn’t be until sunup the next day. 


What Happens To Your Metabolism on a 12-Hour Fast? 

When you restrict your calories for at least 12 hours your body begins to turn stored fat into energy molecules called ketones.

However, this process only starts at around 12 hours. To amplify fat-burning with fasting, extend it to a 16/8 protocol. 

What about Autophagy on 12:12? 

The 12-hour fast is too short to activate the process of cellular renewal known as “autophagy.” 

This process whereby your body breaks down old and damaged cells and recycles their components is one of the most potent and protective benefits of intermittent fasting. 

In order to activate autophagy, you need to fast for between 16-18 hours, minimum. 

It’s also important to note that intermittent fasting will only stimulate significant autophagy when you align your fasting period spans overnight.

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Benefits of a 12-Hour Fast 

Let’s explore the various possible health benefits of a 12-hour fast, including: 

  • Improved body composition
  • Improved insulin sensitivity
  • Increased HGH production
  • Increased BDNF production
  • Reduced inflammation
  • Improved productivity

Weight Loss and Improved Body Composition

As we mentioned above, fasting promotes “fatty acid oxidation,” which is how body fat is turned into energy.

When carbs are absent during the fasting period, insulin drops and norephedrine increases–the recipe for rapid fat burning.  This process promotes weight loss and boosts muscle mass.

Studies have shown that IF can increase your metabolic rate–the speed at which your body burns calories–by up to 14%. This rate increase occurs even when your body is at rest.

Not surprisingly, researchers have found that IF is an effective approach to weight loss, yielding a 9.9% reduction in body weight.

Increased Stem Cell Production and Intestinal Healing

Stem cells are the body’s raw building materials. They’re the foundation of all other cellular structures. 

As we age, our ability to produce stem cells decreases. This can be a particularly detrimental issue in our intestines, where our modern diets subject our intestinal cells to chronic damage from plant toxins, abrasive fiber, and excess carbs. 

With chronic exposure to toxins and a diminished ability to heal and regenerate, our intestinal cells can break down, leaving gaps in our intestinal walls that allow pathogens into the bloodstream. This condition is called intestinal permeability or leaky gut

Once in the bloodstream, these toxic molecules spread throughout the body, contributing to chronic inflammation and autoimmune disorders.

Fasting can stimulate the production of stem cells, providing healing and protection in the gut.   However, to significantly enhance stem cell production, you’ll likely need to fast for longer than 12 hours. 

Improved Insulin Sensitivity

In a wide-ranging 2022 review of many IF studies, researchers found that fasting can be used to treat various diseases linked to insulin resistance.

Using the 12-hour fast to reduce Insulin resistance can contribute to numerous benefits, including

  • Clearer skin
  • less water retention
  • Improvement of diabetic symptoms
  • Lower risk of heart disease
  • Resolution of PCOS
  • Improved cognition and neurological protection

Increased BDNF

Fasting for even short periods of time has been shown to increase an important hormone called brain-derived neurotrophic factor (BDNF-1). 

One study found that fasting from dawn to sunset significantly increased BDFN levels.5

BDFN plays a role in creating new brain cells. That’s why neuropsychiatrist John J. Ratey calls it “Miracle-Gro for the brain.” And it’s associated with improvements in learning and the creation of new neural pathways. 

On the other hand, low BDNF levels have been linked to neurodegenerative diseases like Parkinson’s and Alzheimer’s.

In studies on animals, fasting-induced increases in BDNF are associated with less depression, anxiety, and neurodegenerative disease.

Reduced Inflammation 

Chronic inflammation is a killer. In fact, it’s the most deadly root of disease across the world, afflicting and killing 3 out of 5 people.

Epidemic chronic inflammatory diseases include heart disease, stroke, respiratory diseases, cancer, obesity, and diabetes.

A 12-hour fast can reduce inflammation by

  1. Activating stem cells that help heal intestinal permeability
  2. Reducing levels of inflammatory white blood cells 5

Fasting has been found to be an effective therapy for chronic inflammatory conditions, including rheumatoid arthritis, Sjogren’s, and MS.

12-Hour Fast Concerns

The 12-hour fast is a remarkably gentle approach and is likely safe for most people. 


Restricting calories has effects on the balance of sex hormones in women. However, the 12-hour fast is such a gentle approach that it’s unlikely to adversely affect hormone balance.

Learn more about intermittent fasting for women here

Eating disorders

Intermittent fasting is not recommended for people with eating disorders. It can conceal anorexia and amplify bulimia. 

Foods to Eat On a 12-Hour Fast

What you eat on either side of a 12-hour fast is as important as taking time off from eating. 

The right foods will promote fat metabolism and nourish your fresh crop of stem cells with the nutrients they need to thrive. 

These nutrient-dense whole foods, include

What Can I Drink on a 12-Hour Fast? 

On a 12-hour fast, mineral water is the best choice. 

If you can’t go without black coffee or tea, that’s ok, too. Black coffee contributes only 2-3 calories per cup– not enough to disrupt the fasting metabolic state.

12-Hour Fast: The Bottom Line

The 12 fast is a gentle, often introductory approach to intermittent fasting. 

Intermittent fasting in general, has been shown to promote weight loss, stem cell production, and to stimulate the release of beneficial hormones. 

As with all approaches to fasting, it’s important to listen to your body and make adjustments when necessary.

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