What are the stages of fasting?

Fasting Timer lists seven, from hour 0: blood sugar rises, blood sugar falls, fat burning, ketosis, autophagy, deep autophagy and extended fast. They are approximate, general information. The sources in the table describe the shift from glucose to fat and ketones, but none of them splits it into these seven clock-time stages.

How many hours into a fast does ketosis start?

A 2018 review in Obesity (Anton et al.) says the metabolic switch to fat and ketones typically occurs between 12 and 36 hours after you stop eating, depending on the glycogen in your liver when the fast begins and how much energy you use or exercise during it. The app marks ketosis at hour 12, the early end of that range.

When does autophagy start during a fast?

The sources in this guide don't give an hour. Anton et al. say data suggest fasting stimulates autophagy in muscle cells. The one human study cited here (PLoS One, 2014) took muscle biopsies from 8 healthy men after a 72 hour fast and called the autophagy findings hard to interpret. The hours 18 and 24 in the app are approximate markers. The autophagy guide goes through the studies.

Are the stages of fasting the same for everyone?

No. Anton et al. tie the timing of the switch to the glycogen in the liver at the start of the fast and to energy expenditure or exercise during it, so two people fasting for the same number of hours can be at different points.

Does Fasting Timer show the stages while I fast?

Yes. The Current stage card on the Timer tab and the stage markers on the progress ring are free. Stage alerts, which notify you as you reach a stage, are part of Premium.

Fasting Stages: An Hour-by-Hour Timeline With Sources

Q: Who should check with a doctor before fasting?

Harvard Health's advice on time-restricted eating excludes people with diabetes, people with eating disorders, pregnant or breastfeeding people, and people who need to take medication with food. Fasting Timer also asks anyone under 18 to talk to their doctor first.

The stages of fasting describe how the body's main fuel changes as hours pass since your last meal: first glucose from food, then stored glycogen, then fat and ketones. Fasting Timer names seven stages and marks them at hours 0, 4, 8, 12, 18, 24 and 48: blood sugar rises, blood sugar falls, fat burning, ketosis, autophagy, deep autophagy and extended fast. The hour marks below are the ones Fasting Timer uses. They're approximate, general information, and the sources next to them mostly give a broader picture than a clock time.

Fasting stages timeline

Stage in the app	Starts at	What a cited source says	Source
Blood sugar rises	Hour 0	"After a meal, there is a rise in blood glucose levels, which raises insulin secretion from the pancreas." The same entry says the body brings glucose back to normal within 2 hours.	StatPearls, Physiology, Glucose Metabolism
Blood sugar falls	Hour 4	When glucose falls over the next few hours, the liver releases glucose back into the blood, and glucagon "begins glycogenolysis", the release of glucose from stored glycogen. Low insulin is described as characteristic of fasting. No hour is given for any of this.	StatPearls, Physiology, Glucose Metabolism; Biochemistry, Ketogenesis
Fat burning	Hour 8	The review says that once the metabolic switch is flipped, the main energy source "shifts from glucose to FFA derived from adipose tissue lipolysis and ketones". It places the switch typically 12 to 36 hours after you stop eating, so hour 8 is earlier than this source.	Anton et al., Obesity (2018)
Ketosis	Hour 12	"Low insulin levels", described as characteristic of fasting, activate ketogenesis, and ketone bodies serve as "an alternative energy substrate for peripheral tissues, particularly the brain, during prolonged carbohydrate deficiency". The timing in the review is again 12 to 36 hours, depending on liver glycogen at the start.	StatPearls, Biochemistry, Ketogenesis; Anton et al.
Autophagy	Hour 18	The review says data suggest fasting stimulates autophagy in muscle cells. It gives no hour. In the one human study cited here, 8 healthy men had muscle biopsies after 72 hours of fasting: the autophagy marker LC3B-II rose by about 30%, but p62 also rose by about 10%, which the authors say makes the flux hard to interpret.	Anton et al.; Vendelbo et al., PLoS One (2014)
Deep autophagy	Hour 24	No source here defines "deep autophagy". The only 24 hour figure is that, in the StatPearls entry on fasting, glycogen stores are depleted "after fasting for around 24 hours".	StatPearls, Physiology, Fasting
Extended fast	Hour 48	The same entry says that in more extreme fasting, where fat sources have been expended, the body breaks down skeletal muscle for energy. Anton et al., writing about intermittent fasting in general, note that "safety remains a potential concern".	StatPearls, Physiology, Fasting; Anton et al.

The "Starts at" column holds the app's markers, which aren't measurements. Where the sources give an hour, it's a range (12 to 36 hours for the switch, around 24 hours for glycogen), and they point to the glycogen in the liver at the start as one reason it varies.

What the table doesn't tell you

Your own stage. Anton et al. say the timing depends on liver glycogen at the beginning of the fast and on energy expenditure or exercise during it, so the clock can't tell you where your body is.
Autophagy timing in people. The human data cited here come from one small study of eight men fasting for 72 hours, so the 18 and 24 hour marks are the weakest rows in the table.
Benefits. The table describes fuel and cell processes. Nothing in it says a stage will help you lose weight or improve a condition, and Fasting Timer doesn't claim that either.

Who should check with a doctor first

Harvard Health's advice on time-restricted eating, a daily eating window rather than a long fast, lists exclusions: people with diabetes, people with eating disorders, pregnant or breastfeeding people, and people who need to take medication with food. Fasting Timer's own disclaimer also asks anyone under 18 to talk to their doctor before fasting. If that's you, or you aren't sure, ask first.

If you want a specific length, the guides on a 24 hour fast, a 36 hour fast, a 48 hour fast and a 72 hour fast each go through what the research says for those hours. For shorter daily windows, see the intermittent fasting schedule.

Following the stages in Fasting Timer

Fasting Timer runs on iPhone and shows the same seven stages live. The stage display is free.

Open the Timer tab. Before you start, the stage card is labeled Fasting stages and reads "Start a fast to follow each stage".
Tap Start fasting. The card changes to Current stage with the name of the stage you're in, its short description, and a line such as "Next: Fat burning in" followed by the time left.
The progress ring carries a marker for each stage that falls inside your goal.
Tap the stage card to open Fasting stages, the full timeline. Each row shows when the stage starts ("From the start" for the first one, then "From hour" plus the number), the stage you're in is tagged Now, and the approximate-timings note sits at the bottom. Tap Done to close it.
In Settings, under Notifications, Stage alerts can tell you when you reach fat burning, ketosis and autophagy. That toggle is Premium.

The timer, the daily plans from 12:12 to OMAD, and the stage card are free. Fasting Timer doesn't give medical advice or a diet plan, and your data stays on your iPhone.

Fasting stages are general information, not medical advice. Talk to your doctor before fasting, especially if you are pregnant, have diabetes or another medical condition.

Fasting stages timeline

Fasting stages timeline

What the table doesn't tell you

Who should check with a doctor first

Following the stages in Fasting Timer

FAQ

Sources

More guides