The stages of fasting by hour, from your last meal to hour 72

1. Hours 0 to 4: blood sugar and insulin come down

Say you finish a dinner of chicken, rice and salad at 7 pm on Sunday. That is hour zero. Over the next hour or so your blood sugar rises as the meal is absorbed, and your pancreas releases insulin to move that glucose into your liver and muscles, where part of it is stored as glycogen.

Then both come back down. After a typical mixed meal, blood sugar is usually close to where it started within two to three hours, and insulin follows it down over the next few hours. A big plate of spaghetti with garlic bread and a slice of cake keeps them up longer than a salmon salad does.

This part has been measured in countless meal studies and is as solid as fasting physiology gets. Your body is still storing more fuel than it draws down.

2. Hours 4 to 16: your liver keeps blood sugar steady

By 11 pm digestion is mostly done and your liver takes over. It keeps your blood sugar steady in two ways: by breaking down stored glycogen, and by making new glucose from lactate, glycerol and amino acids, a process called gluconeogenesis.

Popular charts draw these as separate stages, with glycogen first and fat later. The best human data show them overlapping from the start. Researchers at Yale measured liver glycogen every few hours during a 68-hour fast using magnetic resonance spectroscopy. Over the first 22 hours, gluconeogenesis already supplied about 64% of the glucose the body produced. Glycogen supplied the rest, so it was far from empty at hour 12.

How fast your own glycogen falls depends on how much you stored, which goes back to the carbohydrate in your last meals, and on how much you move. A run before breakfast draws on it faster than a morning at your desk.

At 7 am Monday you reach hour 12, and at 11 am hour 16, where the most popular daily schedule ends. If you are still deciding how long your daily fast should be, our guide to intermittent fasting hours compares the common windows.

3. Hours 16 to 24: fat release speeds up and ketones begin to rise

Between late Monday morning and Monday evening comes what charts call the metabolic switch. The clearest hour-by-hour data come from a small University of Texas study in which six healthy young men were measured at 12, 18, 24, 30, 42, 54 and 72 hours without food.

The release of fat from their fat tissue doubled between hour 12 and hour 72. Sixty percent of that increase happened between hours 12 and 24, with the biggest jump between 18 and 24. Insulin fell by about half between hour 12 and hour 72, and 70% of that drop came within the first 24 hours. Blood sugar barely moved between hours 18 and 24, so the surge in fat release tracked falling insulin more closely than any change in glucose.

Your liver turns part of that fat into ketones, so beta-hydroxybutyrate, the ketone that blood meters read, starts to rise somewhere in this window. On day one the levels are usually still low, and how fast they climb depends on what you last ate, how active you are and whether you already eat low-carb.

Six young men are a thin base for anyone else's timetable, so treat 18 to 24 hours as a typical range.

4. Hours 24 to 72: ketosis deepens and your body adjusts

In a Nottingham study, 29 healthy adults, 17 women and 12 men, were measured after 12, 36 and 72 hours without food. Their blood sugar fell and beta-hydroxybutyrate rose at each step, and their fuel mix shifted steadily toward fat. Heart rate went from about 62 to 68 beats a minute by hour 36 and stayed up, while adrenaline and noradrenaline rose by hour 72. Women and men followed the same pattern.

Glycogen fades out during this stretch. In the Yale study, gluconeogenesis supplied 82% of the body's glucose between hours 22 and 36, and 96% between hours 36 and 54. By the third day nearly all of the body's glucose was being made from scratch.

In the Texas study, blood sugar at hour 72 was about 25% lower than at hour 12, roughly 75 mg/dL against 100 mg/dL. That is still a normal fasting value for a healthy adult.

The FastTrack timer shows the phases of your fast up to ketosis. Treat them as a rough guide, since your own ketone curve can run hours off any average.

5. Expect hunger in waves, on your usual meal clock

Many charts show hunger climbing steadily until you eat. The hormone data do not back that up. In a Danish study, 33 young adults, some lean and some with obesity, had blood drawn every three hours from hour 12 to hour 84 of a fast. Ghrelin, the hormone that signals hunger, kept the same daily rhythm the whole time, lowest around 8 am and highest in the afternoon. Its 24-hour average dipped slightly as the fast went on, and the pattern held in men and women, lean or not.

That fits what many people report on a longer fast: the hard moments arrive around their usual lunch and dinner times, then fade. Hunger also runs on habit and on the smell of someone else's toast, which a blood test cannot capture. Our guide to hunger while fasting has practical ways to ride out each wave.

6. Autophagy: the stage nobody has timed in people

Almost every stage chart puts autophagy, the cell's recycling system, at a set hour, usually somewhere between 16 and 24. That hour does not come from human data. In animals, the evidence that fasting switches autophagy on is strong. A 2026 review of the human research found it much thinner. Many studies measured static levels of autophagy genes or proteins, often in blood, which cannot show whether the process is actually running. The few that measured real turnover were often negative or inconclusive.

So there is no validated human timeline for autophagy, and a chart that names an hour is extrapolating from animals or cell studies. It may well rise during longer fasts in people; nobody has yet shown when. Our page on how long to fast for autophagy walks through the few human studies that exist.

7. Check any stages of fasting by hour chart against the data

Before you plan a fast around a chart, hold its claims up to what was measured:

Claim on the chart	What human studies show	Verdict
Insulin drops in the first hours	Settles after each meal, then keeps sliding through day three, most of it in the first 24 hours	Measured
Liver glycogen runs out at hour 12	Still supplied about a third of glucose over the first 22 hours and faded out over days two and three	Too early and too precise
Ketosis starts at hour 12	Ketones start rising within the first day and keep climbing for two to three days	Roughly right, varies a lot
Hunger keeps rising until you eat	Ghrelin keeps its daily rhythm and its average dips slightly over 84 hours	Not supported
Growth hormone rises	Increased in lean adults during an 84-hour fast	Measured in lean adults
Autophagy starts at hour 16 or 24	No validated human timeline; human data are limited	Extrapolated

A rule of thumb: an exact hour for something inside your cells was probably extrapolated, while a range for something in your blood more likely comes from people.

8. Know when to stop and who should skip long fasts

A 16-hour fast is routine for most healthy adults. Fasts past 24 hours ask more of you, and the Nottingham data show it: heart rate and stress hormones rise by day three. Talk to your doctor before any fast if you have diabetes and take insulin or a sulfonylurea such as glipizide, because fasting can push your blood sugar dangerously low. Long fasts are off the table if you are pregnant or breastfeeding, have a history of an eating disorder, or are under 18.

During any fast, these are the signs to act on:

If you notice	What to do
Dizziness when you stand up	Sit down and drink water with a pinch of salt; if it keeps coming back, eat and end the fast
Fainting or nearly fainting	End the fast, eat, and see a doctor even if you feel fine afterward
Palpitations or a racing heart at rest	End the fast; get same-day care if it continues or comes with chest pain
Confusion or unusual weakness	End the fast and get medical help now; check your blood sugar if you have a meter
Shaky and sweaty while on diabetes medication	Treat a low the way your care team showed you, then call them before you fast again

Plan your first fast past 24 hours for a quiet weekend at home, with no hard workouts.

What you can rely on

The first day of a fast is well measured. Insulin falls while your liver leans more on making new glucose, and fat release climbs fastest between hours 18 and 24. Ketones build over days two and three, and hunger keeps to your usual meal clock. Autophagy has no human timeline yet, so any hour you see for it is a guess, and every other hour is an average your body may not match.