36 hour fast stages: what happens hour by hour

The stages of a 36-hour fast, hour by hour: a full day in ketosis, the rise of autophagy after hour 24, what the evidence supports at hour 36, and how to do it with care.

In brief: a 36-hour fast, usually dinner one day to breakfast two days later, passes through the fed state, early fasting, lipolysis and ketosis, then spends about 12 hours in the window where autophagy rises more clearly. Insulin falls from around hour 4, fat release begins near hour 12, ketosis starts around hour 16, and after hour 24 sustained low insulin and mTOR suppression drive cellular recycling. These hours are typical ranges, not thresholds.

A 36-hour fast skips one full day of meals: eat dinner, eat nothing the next day, and break the fast at breakfast the morning after. It sits between a 24 hour fast and a 48 hour fast, and it is the shortest common fast that spends real time past hour 24. The full 0 to 72 hour picture is in the fasting timeline.

What are the stages of a 36 hour fast?

A 36-hour fast spends time in five of the six phases.

Hour of the fast	Stage	What typically happens	Evidence in humans
0–4	Fed state	Insulin is elevated; glucose from the meal is used or stored (Cahill 2006)	Well established
4–12	Early fasting	Insulin falls, glucagon rises, and liver glycogen is drawn down (Cahill 2006; Anton 2018)	Well established
12–16	Lipolysis	Fat cells release free fatty acids; the liver begins making ketones (Cahill 2006)	Well established
16–24	Ketosis	Blood β-hydroxybutyrate rises past about 0.5 mmol/L and keeps climbing (Owen 1967; Newman & Verdin 2017)	Well established; levels vary
24–36	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

What happens in the first 24 hours?

The first day is the same as a 24 hour fast: insulin falls from around hour 4, fat release begins near hour 12, the liver starts making ketones around hour 16, and by hour 24 most people are in steady ketosis. Hunger comes in ghrelin waves at your usual mealtimes and passes within roughly 15 to 45 minutes if you do not eat (Cummings 2001).

What happens between hours 24 and 36?

This is what makes a 36-hour fast different. With insulin 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), and it rises further through the second day. In people, autophagy cannot be measured directly without a tissue biopsy, so the exact hour is an estimate; the honest version is in autophagy, explained. Ketones keep supplying a growing share of the brain's energy, and many people find hunger quieter on the second day than at hour 20.

What should you drink and watch for?

Water, plain tea and black coffee are fine; see what to drink while fasting. Past 24 hours, sodium loss through the kidneys is the usual cause of the dull "fasting headache" and lightheadedness on standing, so most people take sodium, potassium and magnesium during the second day; the electrolyte handbook covers amounts. Faintness, confusion or palpitations are reasons to stop and eat. Break the fast gently, with something small and low in sugar; see how to break a long fast.

Who should be careful with a 36 hour fast?

Treat it as a step up from a few successful 24-hour fasts, not a starting point. Do not attempt it 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, or take regular medication. This page is educational, not medical advice.

Frequently asked questions

What happens during a 36 hour fast?

Over 36 hours the body moves from the fed state through early fasting and lipolysis into ketosis by around hour 16, then spends roughly 12 hours in the window where autophagy rises more clearly, after hour 24. Exact hours vary by person (Cahill 2006; Mizushima & Komatsu 2011).

Is a 36 hour fast enough for autophagy?

It spends about 12 hours past the 24-hour point where the clearer autophagy signal appears in animal studies, so it reaches the window, but how much autophagy happens in a given person cannot be measured directly without a biopsy (Alirezaei 2010).

Is a 36 hour fast better than a 24 hour fast?

It goes further into the autophagy window but is harder, and it needs more attention to electrolytes. Whether it is better depends on your goal, health and tolerance; for weight loss, total intake over the week matters more than the length of one fast.

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. 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
8. Related reading: the fasting timeline by hour, 24 hour fast stages, 48 hour fast stages, 72 hour fast stages, autophagy explained, and how to break a long fast.