Fasting has exploded in popularity. But do you know exactly what your body is doing, hour by hour? If not, how do you know if you’re fasting long enough to reach your goals?

In this article, we answer what are the 5 stages of fasting by hour. And yes, we cover everything from intermittent fasting through up to five days of prolonged fasting.

Here’s the science of the fasting timeline and the benefits you’ll reap in each stage. Plus, we share our own real-time metabolic data to show how our metabolism changes over 5 days… and if it matches up to what the doctors predict.

 

Stages of Fasting By Hour

Stages of Fasting Plus Our Hourly Metabolic Timeline

Whether you’re just starting with intermittent fasting or are interested in a prolonged 5-day fast, let’s jump into the science of what happens to your body during the stages of fasting, hour by hour.

It’s incredibly helpful to know the hourly stages of fasting to help you meet your health goals and decide what to do. Did you fast long enough? Did you make it into fat-burn mode? Can you break your fast now or should you wait awhile longer?

We wanted to write this article because we received many comments and emails about our article, “How to do a 3-Day Fast” from people disappointed that they couldn’t complete all three days.

First off, we congratulate them for listening to their bodies and breaking the fast when they felt that they needed to. Honestly, this is a great skill to have: knowing when (and when not) to push it.

Next, we share the below timeline of stages of fasting by hour because every single person was able to reap benefits from their fast, no matter how long they fasted.

This article is based on scientific studies and our own personal metabolism data, collected with the Lumen metabolism tracker which tracks our metabolism in real time… It’s pretty awesome and we’ll link to our full experience below.

Our goal was to “test” how we stood up against the agreed-upon fasting timeline to see how accurate it was.

Using Lumen, we both tracked a 24-hour water-only fast for this article and also share data that we collected over a previous 5-day fasting-mimicking diet to put us at 130 hours of personal fasting data.

The 5 Stages of Fasting Experiment

The schedule we share below is the one most agreed upon by doctors and researchers, but we want to remind you that everybody is different. It will change based on your nutrition, exercise levels, body composition, stress levels, and more.

What makes our article unique from others is that we’ll be sharing our real-time, metabolic data from our Lumen metabolism tracking devices (10% off with code RYANANDALEX, by the way!) to share how our bodies reacted while fasting.

Spoiler alert: our bodies were fairly far outside the general timeline by as much as 2 hours each way! This just reminds you that being in-tuned with your body and individual needs still remains the number one priority while fasting.

Also, this timeline goes past a standard intermittent fasting protocol. To truly experience the 5 stages of fasting, you need to fast for longer than 72 hours. Therefore, for our readers who don’t finish the 3-day water fast, we sometimes recommend considering a fasting-mimicking diet, like the ProLon 5-Day Fast Program.

A fasting-mimicking diet tricks your body into thinking it’s in a fasted state while you’re still eating. The science is fascinating and we share our experience in our ProLon review… so you have options!

Let’s get to it!

Intermittent and extended fasting bundle - printable pdf guide

The Proven Fasting Bundle (Printable PDF Guide)

Read by 143,364 fasters in 2022 alone, the Fasting Bundle is the structure and tools you need to succeed.

  • Intermittent and alternate-day fasting meal plans
  • Checklist for before, during, and after a prolonged fast
  • Benefits, challenges, and a beginners how-to guide

Plus, we're just an email away for support.

Different Stages of Fasting by Hour

Most doctors and nutritionists agree that there are five stages of fasting. The clock starts from Hour 0 (your last bite of food) and ends… whenever you choose to.

Extended fasting, a popular option, ends at 72 hours or 3 days of fasting. This one is common because you generally don’t need to be under medical supervision to do it or supplement with salts or vitamins. To see how we did it, head over to our 3-day fast article, which includes what we ate to break the fast healthily too.

Our personal data collection for stages of fasting ends at 130 hours, or about 5 and a half days after a fasting-mimicking diet. If you’re ready to undertake this, it’s best to discuss with your doctor about the benefits of prolonged fasting for you specifically.

Here’s a summary of the stages of fasting we’ll talk about:

Stage 1: Hours 0-4 – digestion begins and insulin levels spike
Stage 2: Hours 4-16 – fat-burning mode starts despite hunger pangs
Stage 3: Hours 16-24 – ketosis ramps up as hormones stabilize
Stage 4: Hours 24-72 – autophagy and reduced inflammation
Stage 5: Hours 72+ – immune system regeneration and stem cell production

No matter how long you intend to fast, familiarize yourself with our full guide to fasting to answer your questions on how to conduct a fast safely and what to expect. Better yet, inform your doctor, take notes throughout, and share your experience in the comments.

Finally, a quick note — the fasting we discuss here is a water fast, not a dry fast. This means that you can consume as much water as you’d like, as well as non-calorie drinks like coffee and tea without cream or sugar.

Now let’s dive into what happens to your body while fasting, hour by hour.

Stage 1: Hours 0-4

You’ve just finished your meal and your body gets to work. Your body starts breaking down food, depending on what you’ve just eaten.

Digestion begins and insulin levels spike

Carbs, for example, are broken down into their simple sugar form of glucose. As your blood sugar (aka glucose) levels rise, peaking around 30 minutes after your meal, the pancreas secretes insulin to carry the glucose from the bloodstream to cells to be used for energy. This causes an insulin spike.

About 2 hours after a meal, your blood sugar levels typically return to baseline.

Fats, another main fuel source for the body, are mostly broken down in the small intestine to their simpler chemical building block, triglycerides. Some triglycerides circulate throughout the bloodstream, providing energy to your cells and brain, while most others are stored for future energy use.

Fat is digested slower than carbs, so triglyceride levels peak about 8 hours after a meal.

In the case of both carbs and fats, excess glucose or triglycerides not used for energy are stored in cells for later use… and if your body is never in a fasted state and is continuously being fed, this leads to weight gain.

Protein, meanwhile, is broken down into amino acids, which are used to build and repair cells and muscle. Protein is not typically a fuel source for the body, with carbs and protein covering 95% of the body’s fuel needs. (This is why Lumen shares what your body is burning on in a carbs vs. fat only percentage.)

Your meal’s protein will be fully digested anywhere between 30 minutes to 4 hours, depending on the type of proteins you just ate.

Stage 1 fasting benefits

This first stage of fasting is referred to as the “anabolic stage” because it’s the growth stage of your body. With readily available nutrients, your body can grow, fuel, repair, and store calories for future use.

Stage 1: Our Metabolic Results

During our 24-hour fast, we measured with our Lumen metabolic trackers what our bodies were doing in real-time. Lumen tells us what our body is using for fuel and in what percentages. Our bodies can burn both fat and carbohydrates at the same time but in different proportions, shown like this in the Lumen app:

the best intermittent fasting window to lose belly fat and boost metabolism

While you might be most interested in only fat burn, metabolic flexibility — the ability and ease that your body has to choose to burn either fats or carbohydrates — is important and a key indicator of metabolic health.

Here’s our data from Hours 0-4 (7 pm – 11 pm, after dinner). To us, this indicates that the carbohydrates from our dinner of chicken with rice and broccoli was more or less digested by Hour 3.

Stages of Fasting By Hour Stage 1

Stage 2: Hours 4-16

As your body finishes breaking down the food you ate, you’ve entered the second stage of fasting, the “catabolic stage.”

Catabolism is the process where molecules of stored energy are broken down to use as fuel. This is really what the weight loss folks are after: burning up stored fat.

Fat-burning mode starts despite hunger pangs

Now your last meal has been digested and there’s no more glucose in your bloodstream, so your insulin levels are also down. Your body looks to stored glucose (called glycogen) to use for fuel. Glycogen stores deplete quickly, so this is the tail-end of your carb-burning zone before switching to fat burn.

But it’s not always smooth sailing. After the Hour 4 mark, the effects of leptin — the appetite satiety hormone released during your meal — start wearing off. Another hormone takes its place: ghrelin.

Called the “hunger hormone,” ghrelin is released when your stomach is empty, signaling that it’s time to eat again. This typically happens about 3-4 hours after your last meal, and peaks before mealtime.

Unaware of your fasting plan, ghrelin will rise due to past routines and even circadian rhythm. So, expect to feel rolling waves of hunger during Hours 4-16, especially in the afternoon.

How do you know when you’ve reached a fat-burning state?

If your diet is low in carbohydrates, there’s a good chance that your body has already started burning fat by Hour 16. To know for sure, you’ll need to measure it with Lumen, but there are other indicators too.

When cravings have stopped and you feel a burst of energy, focus, and mood, it’s an indicator that your body has moved on from burning carbs to fat.

Stage 2: Our Metabolism Data

What we’re looking for here is flipping from carb-burn into fat-burn, if the above analysis is right.

Between Hours 4-10 went, we were asleep, but upon waking, both Ryan and I were burning mostly fat.

Great! But… then what happens? Just an hour later, both of us reverted back to carb-burn.

Looking at our data and feeling the hunger pangs, it seemed our body was in stress and continued to burn stored carbs while taking a slow transition into fat-burn mode.

This is shown here with our Hours 4-16 data (11 pm – 11 am for reference):

Stages of Fasting By Hour Stage 2

Stage 3: Hours 16-24

You’ve burned through your glycogen stores and you’re still fasting. Next up: ketosis.

Ketosis ramps up as hormones stabilize

When your body is finished fueling from glucose and glycogen, it turns to burning stored fat. This is referred to as flipping the metabolic switch.

In the process of removing fat from cells to use as energy, the liver releases a chemical called ketones. Ketones travel in the bloodstream to fuel cells and keep your body moving.

Public service announcement: Your body will only burn fat as long as you’re eating enough calories normally. This depends on your activity levels, but a bare minimum for women is 1,200 calories a day and 1,600 for men. If you’re under eating, among other things, your body will break down muscle before fat.

Stage 3 fasting benefits

Research suggests that being in a fasted state and reaching ketosis has a wide range of health benefits. This includes increasing metabolic flexibility (the body’s ability to use both carbs and fat as fuel as needed), promoting weight loss, boosting brain function, improving insulin sensitivity, and even contributing to longevity.

For many people following the 16/8 intermittent fasting plan, the goal is to reach a stage of ketosis. To ensure this happens, keep to a well-balanced diet without overdoing it on the carbs, and you’ll be able to get there before it’s time for breakfast.

By the way, if the name “ketosis” sounds familiar, it’s because it’s the basis for the keto/ketogenic diet — something we don’t support, by the way. We never support a restrictive or low-carb diet.

Stage 3: Our Metabolism Data

Hours 16-24 (our 11 am – 7 pm) were marked by huge swings in burning fat and burning carbs. Per Stage 3, we would expect to be squarely in the fat-burn zone, but our numbers don’t show this.

Why? Are the doctors wrong?

Before throwing in the towel, remember that metabolism is a function of several things, including nutrition, stress, movement, and sleep. The day we measured was a stressful and high-movement day as we moved apartments and loaded up a big moving truck for hours.

Our fat burn scores started out strong, but after a stressful day we felt depleted and our Lumen metabolism scores reflected that.

So simply making it to Stage 3 doesn’t mean it’s a home run for fat burn, with your lifestyle choices still making up a large part of it.

Stages of Fasting By Hour Stage 3

Stage 4: Hours 24-72

After fasting for 24 hours, your body has likely completely digested its last meal and then some. That means the body’s resources and energy can be put into other jobs instead of digesting your last slice of pizza: like cellular repair and destroying pathogens.

At this stage, your body ramps up a process called autophagy as much as 300% between hours 24-72.

A quick note. We’ve done a 3-day water fast before and it’s eye-opening… I only wish we had Lumen at the time to share our data!

The good news is that we did have our Lumens for a 5-day fasting-mimicking diet, and we’re going to share our results from that here with you instead.

Autophagy and Reduced Inflammation

Autophagy is when cells “clean house” — they recycle old and damaged parts while decluttering and disposing of non-functioning parts. It also destroys any harmful bacteria and viruses found in the cells.

Studies show that autophagy is typically induced around 24-48 hours into fasting. It can also occur after calorie restriction and intense exercise. So while autophagy is often touted as a fountain of youth, the research is still mixed.

The Latin origin of autophagy means “self-devouring.” Essentially, when your body is out of fuel and resources, cells eat themselves to survive by upcycling and regenerating old parts to keep the wheels turning.

So, overdoing it is inadvisable. This is why we recommend doing extended fasts only once a quarter or less.

Stage 4 fasting benefits

However, in small doses, fasting and autophagy have been linked to reduced cancer risks, higher longevity, and stabilized hormone levels.

In addition to autophagy, the fourth stage of fasting includes increased levels of human growth hormone and brain-derived neurotrophic factor (BDNF).

Human growth hormone is why you won’t lose muscle during an extended fast while lowering levels of cortisol and stress.

Brain-derived neurotrophic factor promotes brain plasticity, neural connections, and repairs and protects brain cells. This is why during the fourth stage you may experience yet another surge of sharp focus and memory. Not only does your brain run seamlessly off of ketones, but you’re also giving brain cells time to cleanse and repair.

At this stage, blood sugar and insulin levels remain low as disease-causing inflammation plummets. Ghrelin, the hunger hormone, also remains low. So while ghrelin follows a circadian rhythm, studies show that after every 24 hours of fasting, ghrelin production slows down further and further.

Stage 4: Our Metabolism Data

Unfortunately, the last time we did a 3-day water fast, we did not have Lumens yet. However, we did track our data while completing ProLon’s 5-day fasting-mimicking diet.

As introduced earlier, a fasting-mimicking diet “tricks” the body into thinking that you’re on a fast when, in reality, we ate between 700-1100 calories a day. This is done through a combination of eating mainly a high fat diet low in calories, carbohydrates, and protein. With ProLon, all food was provided in their meal kit and it is formulated to mimic that of a full water fast.

Our Lumen metabolism data showed that, for the vast majority of measurements, we were both squarely in fat-burn mode, just as you would hope for after 24 hours of fasting (even if mimicking!)

Stages of Fasting By Hour Stage 4

Stage 5: 72+ Hours

Once you hit the 72-hour mark, you’ve left the realm of extended fasting and entered prolonged fasting. This typically wraps up by Day 5, or at the 120-hour mark. Wow!

While we’ve never done a 5-day water fast before — again, medical supervision is advised — we have done a 5-day mimicking-fasting diet with ProLon and seemingly received all of the same benefits without any of the downsides. The downsides usually are whacked out sleep and strong hunger.

This is pretty much the only way we recommend reaping the fasting benefits through five days… and it’s shockingly easy to follow since all food is provided.

Fasting-Mimicking Diets

A fasting-mimicking diet is when you reduce your caloric intake and eat certain foods that keep your body thinking it’s in a fasted state… although, you’re eating between 700-1100 calories a day so you don’t get the hunger pains, depleted energy, or whacked-out sleep that usually comes with long fasts.

Developed by the world’s leading expert in fasting, Dr. Valter Longo, alongside the Longevity Institute of the University of Southern California, decades of research went into creating ProLon’s 5-Day Fasting Program — the only fasting-mimicking diet meal plan with clinical trials on the market.

For more information, we highly recommend Dr. Longo’s book and checking out our experience, before-and-after results, and metabolism data in our ProLon review.

With ProLon, we got all of the food we ate for five days delivered to us and saw some pretty amazing results — similar to the results we saw when we water-fasted but without any of the hardship. If you’re interested, ProLon is offering 15% off with our link and code RYANANDALEX.

Alright, now that we’ve shared how you can get to 72+ hours of fasting, let’s get into the benefits associated with that stage of fasting.

Immune system regeneration and stem cell production

As far as what happens to your body while fasting during the fifth stage, all of the above benefits continue in a state of deep ketosis. Continued fat-burning, reduced inflammation, and stabilized hormone levels are all seen.

Autophagy will also continue but the rate will plateau and increase only about 30% more before topping off.

Stage 5 fasting benefits

Research shows the main difference between Stages 4 and 5 of fasting is that between the 3-5 day mark, there’s evidence of increased stem cell production and nearly complete immune system regeneration.

Studies have also found cell resistance to stress and toxins after 72 hours.

If you continue on this route, you’ll see amplified results for fasting and continue to reap long-term benefits. Of course, it’s important to continue listening to your body. Be ready to end the fast at any time if you feel weak or sick, and consider adding a pinch of sea salt to your water when doing a prolonged DIY fast or anything longer.

Stage 5: Our Metabolism Data

Our metabolism data continues into Stage 5 to round out the stages of fasting by hour. Again, we are sharing data from when we did the ProLon fasting-mimicking diet, so we were not on a pure water fast.

Our Lumen metabolism trackers continued to show that we were both in fat-burn nearly exclusively from Hours 72-130. This corroborates all of our research, but you’ll still see dips of fat burn vs. carb burn. Is this because we were simply mimicking fasting, or were our bodies under more stress?

Honestly, we’re not sure and we can’t measure the truly good stuff (aka, our prolonged fast’s impact on longevity). But, anecdotally, we can share that we still measured some amazing benefits from our 5-day fast which would point to it working as hoped.

This includes our digestive tracts cleaning house, feeling energized, and Ryan losing 9.6 lbs. (from a starting weight of 165.8 lbs.) and Alex losing 5.4 lbs. (from a starting weight of 106.2 lbs.)

As for measuring the truly good stuff, like a prolonged fast’s impact on our longevity… I guess we’ll just have to wait and see!

Stages of Fasting By Hour Stage 5

Closing Thoughts on the 5 stages of fasting

The 5 stages of fasting by hour show what happens to your body once the timer starts after your last meal.

Stage 1, or Hours 0-4, is characterized by digestion and insulin spikes as your body gets to work digesting the food you just ate. Depending on what your last meal was, your food should be fully digested around 4 hours later (but sometimes as long as 8 hours.)

This is the stage where your body is taking nutrients and actively using them for energy, repairing cells, or storing them.

The next stage, Stage 2, is the catabolic, or breakdown, stage. Now that your body has been without food, Hours 4-16 are marked by the breakdown of stored energy, including the fat stored in adipose tissues. This is where the sweet spot is for intermittent fasters working towards weight loss.

Ketosis is the name of the game in Stage 3, from hours 16-24, where you should be in full-fledged fat-burn mode. However, I say “should be” because other factors, such as sleep, stress, and movement also affect metabolism.

In our case, our stressful day caught up to us and our bodies were switching between fat-burning and carb-burning. It’s not a bad thing, just note that shorter fasts will see these swings until your carb stores are fully depleted.

Next, Stage 4 from Hours 24-72 is when you can expect autophagy, or cellular regeneration, to begin. And of course, consistent fat burn.

The final stage, Stage 5 of 72+ hours, continues the above benefits from Stage 4. Studies also suggest that stem cells start being produced and that your whole immune system will regenerate during this time.

Our Lumen data during our 5-day fasting-mimicking diet for Stages 4 and 5 have us in fat burn nearly exclusively, but it still fluctuated a bit. I’d say that everything seemed to fit the timeline we researched, though.

All in all, use this stages of fasting by hour timeline as a rule of thumb measure as you consider your next fast and the health benefits that you want to achieve.

Better yet, copy what we did here for yourself and track your metabolism in real-time with Lumen (10% off with code RYANANDALEX). This is really the only definitive way to know if you’ve fasted long enough to meet your goals when it comes to carb-burn vs. fat-burn.

Finally, download the free Fasting Bundle below to help you plan, track, and succeed with both intermittent and prolonged fasting.

For more resources and guides about fasting, such as our guide to intermittent fasting and sleep, head over to our Fasting Page.

Intermittent and extended fasting bundle - printable pdf guide

The Proven Fasting Bundle (Printable PDF Guide)

Read by 143,364 fasters in 2022 alone, the Fasting Bundle is the structure and tools you need to succeed.

  • Intermittent and alternate-day fasting meal plans
  • Checklist for before, during, and after a prolonged fast
  • Benefits, challenges, and a beginners how-to guide

Plus, we're just an email away for support.

30-Day Weight Loss Challenge Tracker Form V2 Ryan and Alex Duo Life

30-Day Weight Loss Challenge

As engineers with a combined twelve years of health coaching experience, we needed to create a data-driven way for our clients to sustain weight loss.

Too many weight loss challenges involve a long list of what you can, cannot, and need to do every day. We’ve reduced the overwhelm and only require you to track five numbers a day — calories, steps, fiber, sleep, and waistline.

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As Seen In Feature Bar Ryan and Alex Duo Life

Hey we're Ryan and Alex

A husband-wife duo, two engineers, and the creators of Ryan and Alex Duo Life. 

After eight years working in the corporate world as engineers, we left to tackle our true passion:

Helping highly motivated couples optimize their relationship and health by cutting through the muck and sharing what the research says works.

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