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 go to three days of fasting or more if you’re interested in extended or 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 24 hours… and if it matches up to what the doctors say.

Stages of Fasting By Hour

Stages of Fasting Plus Our Hourly Metabolic Timeline

Whether you’re just starting out with intermittent fasting or are interested in an extended 3-day fast, 5-day fast, or longer, 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 to ketosis? Can you break your fast now or should you wait awhile longer?

We wanted to write this article because we receive 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 needed to.

Next, we share the below timeline of stages of fasting by hour because every single person was able to reap benefits from their fast.

Lastly, if you’re wondering how long you should be intermittent fasting based on your age or gender or fitness goals, please don’t listen to the advertisements recommending a tailored plan. They’re not based on any research.

If you want a fasting window recommendation based on the good science, see our article below. 

The 5 Stages of Fasting Experiment

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 (15% off with code RYANANDALEX).

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!)

The schedule we share below is the one most agreed upon by doctors, 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 is that we’ll be sharing our real-time, metabolic data from our Lumen metabolism tracking devices to share how our bodies reacted while fasting.

Spoiler alert: our bodies were 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.

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.

Our timeline for stages of fasting ends at 72+ hours, but there’s plenty of documentation on the health benefits of fasting that extends through Day 5 and beyond. For this, it’s best to discuss with your doctor about the benefits of prolonged fasting for you specifically.

Or, as we mentioned above, you could do a prolonged fast like we did with ProLon, a fasting-mimicking diet where you eat food throughout so don’t actually need supervision while reaping the benefits of a 5-day fast.

Here’s a summary of the stages of fasting:

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 (particularly an extended 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 your 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.

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.

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 for metabolic health.

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

Stages of Fasting By Hour Stage 1 Our Metabolic Data

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, 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 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 meal, and peaks before mealtime.

Unaware of your fasting plan, ghrelin will rise due to past routine 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 a home metabolism tracker like Lumen, but there are other indicators too.

(By the way, as of January 2024, Lumen metabolism trackers are $80 off using our link and code RYANANDALEX).

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

If you’re wondering where Hours 4-10 went, we were asleep.

Upon waking, both Ryan and I were burning mostly stored carbs — something that continued throughout the day until Hour 16 (our 11am, for reference).

What’s interesting is that, at Hour 11, I (Alex) was burning mostly fat. This is normal for me, and I know after a long, restful night’s sleep I wake up in fat burn. I’ve learned over time that once the hunger pangs start soon after, it coincides with carb burn. This is shown here:

Stages of Fasting By Hour Stage 2 Our Metabolic Data

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.

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), 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.

The only way to truly confirm, however, is to use a metabolic tracking device, like Lumen, which will tell you if you’re burning 80% fat and 20% carbs as an example, or to measure your ketones using a urine test strip like these.

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 Fasting Timeline Data

Hours 16-24 (our 11am – 7pm for reference) 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 metabolism reflected that.

Stages of Fasting By Hour Stage 3 Our Metabolic Timeline

Stage 4: Hours 24-72

After fasting for 24 hours, your body has 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. Just know that the general recommendation is that this is as long as you should fast by yourself without medical supervision. It is possible to go longer on your own (read more below), but most will stop here.

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 neutrophic 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 neutrophic factor promotes brain elasticity, 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.

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. This is pretty much the only way we recommend reaping the fasting benefits through five days… and it’s shockingly easy and we didn’t get hungry.

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 Stage 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 longterm 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.

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 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 is the catabolic, or breakdown, stage. Now that your body has been without food, Hours 4-16 are marked by the break down 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. While we didn’t measure or fast to Hour 72, our last experience brought us mental clarity, strength, and no hunger as our bodies cleaned shop.

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.

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.

Feel free to download the 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.

You’ll track five numbers daily to give you the highest return for sustainable weight loss. As a bonus, we’ll share our Weight Loss Bundle, which includes strategies, progress tracking tools, and additional weight loss plans.

Learn more about the program.

As Seen In Feature Bar Ryan and Alex Duo Life

Hey we're Ryan and Alex

The creators of Ryan and Alex Duo Life. We are a husband-wife duo and “lifestyle engineers.”

After eight years working in the corporate world as engineers, we left our high-powered jobs to tackle our true passion — helping couples engineer their best lives.

The synergy of our engineering minds and ten years of health coaching experience produced Ryan and Alex Duo Life. Our mission is to help you transform your bodies, minds, and relationship as a couple.

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