72 hour fast stages: what happens hour by hour

The stages of a 72-hour fast, hour by hour: how the body moves from ketosis into rising autophagy and the deep fast, what the evidence supports at each point, and why this is a fast to prepare for, not improvise.

In brief: a 72-hour fast passes through all six phases of fasting. Insulin falls from around hour 4, fat release starts near hour 12, ketosis begins around hour 16, autophagy rises more clearly after hour 24, and from hour 48 the body is in the deep fast: growth hormone is up and ketones supply most of the brain's energy. These hours are typical ranges, not thresholds, and a three-day fast is not for everyone.

A 72-hour fast is three full days without food, usually water, plain tea, black coffee and electrolytes only. It is the longest fast most people should consider without medical supervision, and it is the only common protocol that spends real time in the last two phases of the fasting timeline. Where a 16 hour fast touches ketosis and a 20 hour fast spends a few hours in it, a 72-hour fast lives in ketosis for more than two days.

What are the stages of a 72 hour fast?

A 72-hour fast spends time in all six phases. The first day is the same as any daily fast; the second and third days are where it becomes different.

Hour of the fast	Stage	What typically happens	Evidence in humans
0–4	Fed state	Insulin is elevated; glucose from the last meal is used or stored as glycogen and fat (Cahill 2006)	Well established
4–12	Early fasting	Insulin falls and glucagon rises; the liver releases its own glycogen, which is substantially drawn down by hours 8 to 12 (Cahill 2006; Anton 2018)	Well established
12–16	Lipolysis	Fat cells release free fatty acids; the liver starts converting some into ketones (Cahill 2006)	Well established
16–24	Ketosis	Blood β-hydroxybutyrate crosses the 0.5 mmol/L threshold and keeps rising; ketones become a brain fuel (Owen 1967; Newman & Verdin 2017)	Well established; levels vary by person
24–48	Autophagy rising	Sustained low insulin and mTOR suppression drive cellular recycling, with the clearer signal after about hour 24 in animal studies (Alirezaei 2010; Mizushima & Komatsu 2011)	Mechanism strong; human evidence limited
48–72	Deep fast	Growth hormone rises over baseline, helping preserve lean mass; ketones now supply most of the brain's energy (Ho 1988; Owen 1967; Cahill 2006)	Established for ketones and growth hormone; cellular-cleanup claims preliminary

Safety first. This page is educational, not medical advice. Do not attempt a 72-hour fast if you are pregnant or breastfeeding, have type 1 diabetes, take insulin or other glucose-lowering medication, have a history of an eating disorder, are under 18, or are significantly underweight. Talk to your doctor first if you have type 2 diabetes, kidney, heart or thyroid disease, gout, a history of gallstones, or take regular medication.

What happens in the first 24 hours?

Day one is the familiar part. Insulin falls from around hour 4, liver glycogen runs low by hours 12 to 16, fat release begins, and the liver starts making ketones around hour 16. By hour 24 most people are in steady ketosis, with blood β-hydroxybutyrate commonly reported around 0.5 to 1.5 mmol/L, though individual levels vary widely. Hunger comes in ghrelin waves tied to your usual mealtimes and fades within roughly 15 to 45 minutes if you do not eat (Cummings 2001).

What happens between hours 24 and 48?

This is where autophagy, the cell's recycling program, rises more clearly. With insulin low and mTOR suppressed for a sustained stretch, cells package damaged proteins and worn-out organelles and break them down for reuse (Mizushima & Komatsu 2011). In animal studies the signal in liver and brain is clearer after about 24 hours (Alirezaei 2010). In people, autophagy cannot be measured directly without a tissue biopsy, so the exact hour is an estimate. The honest version of the evidence is in autophagy, explained.

Day two is also when sodium matters. Low insulin makes the kidneys excrete more sodium, and past 24 hours that loss is the usual cause of the dull "fasting headache" and lightheadedness on standing. Most people on a multi-day fast take roughly 2 to 3 g of sodium a day, about a teaspoon of salt spread across the day, plus potassium and magnesium; the electrolyte handbook covers doses and forms.

What happens between hours 48 and 72?

Past hour 48 you are in the deep fast. Growth hormone rises substantially over baseline, which helps preserve lean mass against the pressure of going without food (Ho 1988). Ketones now supply the majority of the brain's energy rather than a share of it (Owen 1967; Cahill 2006), and many people describe hunger as quieter on day three than on day one. Autophagy is at its most active in this window, although the human evidence for specific cellular benefits remains preliminary.

Is a 72 hour fast worth it compared with a 24 or 48 hour fast?

It depends on the goal. For weight loss, a long fast adds little that a daily schedule does not: the scale drop after three days is largely water and glycogen, and calorie-matched trials find fasting about as effective as ordinary calorie restriction (fasting and weight loss). The main reason people choose 72 hours is to spend time in autophagy and the deep fast. Beyond 72 hours the marginal return falls while the physiological cost keeps rising, which is why few people should go further without medical supervision.

How should you break a 72 hour fast?

Gently. After three days the gut and pancreas have downshifted, so a large or sugary first meal often brings bloating, nausea or a drowsy crash. Break it with something small and low in sugar, such as bone broth, eggs or cooked vegetables, stop before you are full, and wait one to two hours before eating more. Refeeding syndrome is a real risk after more than five days of starvation or in malnutrition, not a meaningful one for a healthy adult after 72 hours, but the same "small, simple, slow" rule applies. The full guide is how to break a long fast.

Frequently asked questions

What happens during a 72 hour fast?

Over 72 hours the body moves through all six phases: the fed state, early fasting, lipolysis around hour 12, ketosis from around hour 16, rising autophagy after hour 24, and the deep fast from hour 48, when growth hormone rises and ketones supply most of the brain's energy. Exact hours vary by person (Cahill 2006).

When does autophagy start during a 72 hour fast?

Autophagy begins to pick up in the ketosis window but rises more clearly after about 24 hours, and it is at its most active between roughly 36 and 72 hours. In people it cannot be measured directly without a biopsy, so these hours come from animal studies and indirect markers (Alirezaei 2010; Mizushima & Komatsu 2011).

Do you lose muscle on a 72 hour fast?

Some protein is broken down during any long fast, but the body works to limit it: growth hormone rises over baseline and ketones replace much of the glucose the brain would otherwise need, which reduces the need to make glucose from protein (Ho 1988; Cahill 2006). Eating enough protein before and after the fast also helps.

How much weight do you lose in a 72 hour fast?

Often a few kilograms on the scale, but most of that is water and glycogen rather than fat, and much of it returns within days of eating normally. Fat loss over three days is far smaller than the scale suggests.

Sources

1. Cahill GF Jr. "Fuel metabolism in starvation." Annu Rev Nutr, 2006. doi:10.1146/annurev.nutr.26.061505.111258
2. Owen OE, Morgan AP, Kemp HG, Sullivan JM, Herrera MG, Cahill GF Jr. "Brain metabolism during fasting." J Clin Invest, 1967. doi:10.1172/JCI105650
3. Newman JC, Verdin E. "β-hydroxybutyrate: a signaling metabolite." Annu Rev Nutr, 2017. doi:10.1146/annurev-nutr-071816-064916
4. Anton SD, Moehl K, Donahoo WT, et al. "Flipping the metabolic switch: Understanding and applying the health benefits of fasting." Obesity (Silver Spring), 2018. doi:10.1002/oby.22065
5. Mizushima N, Komatsu M. "Autophagy: renovation of cells and tissues." Cell, 2011. doi:10.1016/j.cell.2011.10.026
6. Alirezaei M, Kemball CC, Flynn CT, Wood MR, Whitton JL, Kiosses WB. "Short-term fasting induces profound neuronal autophagy." Autophagy, 2010. doi:10.4161/auto.6.6.12376
7. Ho KY, Veldhuis JD, Johnson ML, et al. "Fasting enhances growth hormone secretion and amplifies the complex rhythms of growth hormone secretion in man." J Clin Invest, 1988. doi:10.1172/JCI113450
8. Cummings DE, Purnell JQ, Frayo RS, Schmidova K, Wisse BE, Weigle DS. "A preprandial rise in plasma ghrelin levels suggests a role in meal initiation in humans." Diabetes, 2001. doi:10.2337/diabetes.50.8.1714
9. Related reading: the fasting timeline by hour, 16 hour fast stages, 20 hour fast stages, 24 hour, 36 hour and 48 hour fast stages, autophagy explained, the electrolyte handbook, and how to break a long fast.