Stages of Fasting: Hour-By-Hour Timeline

While fasting, the body enters time-sensitive states and phases that change by the hour. Below, we look at the fasting timeline and what happens during a fast.

What Are the 6 Stages of Fasting?

The following are the 6 phases of fasting:

  1. During the 1st stage, your body is still absorbing nutrients from previous meals and blood sugar is rising.
  2. In the 2nd stage, blood sugar drops.
  3. In the 3rd stage, blood sugar balances out to normal levels.
  4. The 4th stage is the fat burning stage, 16 hours into a fast.
  5. Ketosis peaks in the 5th phase, breaking down fat stores for energy.
  6. In the 6th phase, autophagy and cell repair may be triggered from the process of ketosis.

How Does Fasting Affect Us Hour-By-Hour?

Let’s look at how fasting can affect our systems hourly.

0-4 Hours Fasting

  • High blood sugar
  • The first four hours after eating, the body is absorbing nutrients and storing glycogen and fat.

4-8 Hours into Fasting

  • Low blood sugar
  • Glycogen fuels the body with energy. It helps regulate blood sugar in the body.
  • The body begins to break down stored food.
  • Breakdown diminishes insulin and glucose levels.
  • Without insulin, blood sugars fall.
  • At the end of this stage, the stomach is nearly empty.

8-16 Hour Fasting

  • Normal blood Sugar
  • Food has been burned.
  • The body initiates its first healing process.
  • Human Growth Hormone (HGH) begins to rise.
  • Glucagon, another hormone, is released by the pancreas to balance blood sugars.
  • The body is still consuming glucose and hasn’t yet converted to fat.
  • Partial ketosis takes place twelve to fourteen hours after ingesting food.
  • The body will soon run out of glucose, potentially triggering a switch to Ketosis.

16-18 Hours of Fasting

  • Partial Ketosis
  • The body will begin using its stored fat as energy.
  • Your HGH continues to rise during this time.
Is it better to fast 16 or 18 hours?

An 18 hour fast, as studies have shown, is more likely yielding results If you’re trying to lose weight, burn fat, and build muscle.

18-20 Hours Fasting

Most people choose to break their fast at around 18 hours, however, active ketosis only occurs 18-36 hours after starting a fast. During this time what happens?

  • Fat burning and Ketosis
  • The body starts ramping up fat burning.
  • Glycogen stores in the liver have been depleted, and your body begins breaking down protein and fat stores for energy instead.
  • At around 18 hours, the fasting state officially begins and ketosis will last about 2 days, after which ketones are raised even higher if the fast continues.
  • During this time, the liver is turning fat stores into ketone bodies that will be the new fuel for the muscles, heart, and brain.
  • HGH will continue to rise.
  • The body begins to detoxify through autophagy, or “self-eating”, and recycles its damaged or old cells.

20-24 Hours Fasting

  • Peak of ketosis
  • Fat burning continues.
  • The body begins resetting its entire immune system at this point, renewing its good stem cells and replacing old, dying, and dysfunctional cells.
  • All glycogen stores are drained, ketones are released into the blood stream.
  • Glucose supplies are likely gone, while the body begins to rely on Ketones for energy.

Day 1 Results

After an entire 24-hour day of fasting, the body has gone through a state of cleansing, entering into a fasting state and breaking down its stores of glycogen.1

36 Hour Fasting Timeline

Your body should have entered and maintained a state of ketosis after 36 hours.

  • The body’s cleanup system begins to work overtime if it wasn’t already.
  • Cells are being completely repaired.
  • Body is now running on stored energy in the form of ketones.

48 Hours

  • 48-hour fasting is the longest “mainstream” intermittent fasting duration.
  • Autophagy strengthens and repairs cells at an even more accelerated rate.
  • Immune system should be fully reset, so you may experience a noticeable reduction in inflammatory response.
  • Some patients showed a 5 fold increase in HGH in response to a 2 day fast.2

Fasting for more than 2 days will often cause feelings of weakness and lightheadedness as the body adjusts to using ketones and muscle protein instead of glucose for energy.

60-72 Hours Fasting

  • The body is still repairing itself through the process of accelerated autophagy.
  • At this point, you would have plenty of ketone-fueled energy and mental clarity, along with a brand new immune system.

To extend your fast beyond this point, in fact to begin a fast at all, make sure you stay hydrated and always consult with your doctor first.

72-96+ Hours Fasting

  • Intestinal stem cells max out.
  • A microbiome reset.
  • Human growth hormone levels reset.
  1. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3248697/
  2. https://pubmed.ncbi.nlm.nih.gov/1548337/
Scroll to Top