24 hour fast stages: what happens hour by hour

The stages of a 24-hour fast, hour by hour: from the last meal through lipolysis and ketosis to the start of the autophagy window, what the evidence supports at hour 24, and who should take care.

In brief: a 24-hour fast, usually dinner to dinner, passes through the fed state, early fasting, lipolysis and ketosis, and ends at the start of the autophagy window. Insulin falls from around hour 4, fat release begins near hour 12, ketosis starts around hour 16, and by hour 24 blood ketones are commonly around 0.5 to 1.5 mmol/L. Autophagy is only beginning to rise more clearly at this point. These hours are typical ranges, not thresholds.

A 24-hour fast is one full day without food, most often from one evening meal to the next, so you still eat once each calendar day. It is the first of the extended fasts and a common weekly rhythm for people who already do 16:8 or 20:4. This page walks the clock; the full 0 to 72 hour picture is in the fasting timeline.

What are the stages of a 24 hour fast?

A 24-hour fast spends time in four of the six phases and ends at the edge of the fifth.

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 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
16–20	Ketosis begins	Blood β-hydroxybutyrate crosses about 0.5 mmol/L and keeps rising; ketones become a brain fuel (Owen 1967; Newman & Verdin 2017)	Well established; timing varies
20–24	Deepening ketosis	Ketone levels keep climbing, commonly about 0.5–1.5 mmol/L by hour 24, with wide individual variation; autophagy is starting to rise (Alirezaei 2010)	Ketones well supported; autophagy evidence mostly animal

What happens in the first 16 hours?

The first two thirds of a 24-hour fast are the same as a daily 16:8 fast. Insulin falls from around hour 4, the liver draws down its glycogen, fat release begins near hour 12, and the liver starts making ketones around hour 16. Hunger arrives in ghrelin waves at your usual mealtimes and fades within roughly 15 to 45 minutes if you do not eat (Cummings 2001).

What happens between hours 16 and 24?

This is the part a daily fast does not reach. With liver glycogen low and insulin at a low baseline, the liver turns more fatty acids into β-hydroxybutyrate, which crosses the blood-brain barrier and becomes fuel for neurons (Owen 1967; Newman & Verdin 2017). Ketone levels keep rising through the evening, and many people report that hunger is quieter in the last few hours than at hour 12. The mechanism is explained in how the body enters ketosis.

Does a 24 hour fast trigger autophagy?

It reaches the start of it. Autophagy is driven by sustained suppression of mTOR and insulin, and its clearer signal in animal studies of liver and brain appears at about 24 hours (Alirezaei 2010; Mizushima & Komatsu 2011). A 24-hour fast ends right there, so it touches the window rather than spending time in it. In people, autophagy cannot be measured directly without a tissue biopsy. The longer 36 hour and 48 hour fasts are where it rises further.

What should you drink and watch for?

Water, plain tea and black coffee do not meaningfully break a fast; see what to drink while fasting. By the end of a full day sodium starts to matter, because low insulin makes the kidneys excrete more of it; a pinch of salt in water is often enough for a 24-hour fast, and the electrolyte handbook covers longer fasts. Faintness, confusion or palpitations are reasons to stop and eat.

Who should be careful with a 24 hour fast?

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 24 hour fast?

Over 24 hours the body moves from the fed state through early fasting and lipolysis into ketosis. Insulin falls from around hour 4, fat release begins near hour 12, ketosis starts around hour 16, and by hour 24 ketone levels are higher and autophagy is beginning to rise. Exact hours vary by person (Cahill 2006).

Are you in ketosis after 24 hours of fasting?

For most people, yes. Ketone production usually begins around hour 16, and by hour 24 blood β-hydroxybutyrate is commonly around 0.5 to 1.5 mmol/L, although individual levels vary widely (Newman & Verdin 2017).

How often can you do a 24 hour fast?

Once a week is a common rhythm for people who already fast daily. It is a step up from a daily schedule, not a starting point, and it depends on your health and how you tolerate it.

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. 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
6. Mizushima N, Komatsu M. "Autophagy: renovation of cells and tissues." Cell, 2011. doi:10.1016/j.cell.2011.10.026
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, 20 hour fast stages, 36 hour fast stages, 48 hour fast stages, 72 hour fast stages, and what to drink while fasting.