Fasting timeline: what happens hour by hour (0–72h)

A walk down the clock of a fast — the six biological phases your body passes through, when each one arrives, and how far a given fast actually reaches.

In brief: a fast moves through six biological phases on a predictable clock. Insulin falls around hour 4, fat-burning lipolysis begins near hour 12, ketosis is usually reached by hour 16, and autophagy ramps up after hour 24 — peaking only in the 48–72-hour deep fast. The phase you reach depends on how long you go.

Ask "what happens to your body when you fast?" and the honest answer is: it depends on the hour. A fast is not one state — it's a sequence. The body handles hour 2 very differently from hour 20, and the shifts happen on a clock that's roughly the same from person to person. This is the fasting timeline: the same six phases, in the same order, every time.

This article walks the clock from your last bite to hour 72. It uses the same six-phase model as our definition pillar on intermittent fasting, and links each phase to the deeper article on its biology. Reaching a phase isn't automatic — it depends on how long you fast — so the protocol you pick decides which of these stages you'll actually visit.

What are the stages of intermittent fasting?

Every fast — whether 14 hours or 72 — walks through the same six stages, and the only thing that changes is how far down the list you get. The table below is the whole timeline at a glance; the sections after it explain each phase.

Phase	Hours	What's happening	Read more
Fed state	0–4	Insulin is elevated; glucose is used or stored as glycogen and fat	—
Early fasting	4–12	Insulin drops, glucagon rises; liver glycogen becomes the main fuel	The ghrelin wave
Lipolysis	12–16	Glycogen depletes; fat cells release free fatty acids	What breaks a fast
Ketosis	16–24	The liver converts fatty acids to ketones the brain can use	How the body enters ketosis
Autophagy	24–48	mTOR suppression drives cellular recycling	Autophagy, explained gently
Deep fast	48–72	Growth hormone rises; ketones power most of the brain	How to break a long fast

These boundaries are a calibration, not a stopwatch — individual metabolism, your last meal, and your activity all shift the exact hour. But the order never changes, and the rough timings hold across the literature on starvation metabolism (Cahill 2006).

Hours 0–4: what happens in the fed state?

For the first few hours after a meal, you're in the fed state: blood glucose is up, insulin is up with it, and glucose is pouring into muscle and liver cells to be burned now or stored as glycogen. Anything left over is stored as fat in adipocytes (Cahill 2006). Nothing about fasting has really started yet — the body is still running on the food that just arrived. Fat-burning and ketone production are switched off, because insulin keeps them switched off.

Hours 4–12: what happens in early fasting?

About four hours after your last meal, insulin starts to fall — the quietest and most important switch in the whole timeline. Glucagon rises to balance it, and the liver begins breaking down its own glycogen to keep blood glucose steady (Cahill 2006; Anton 2018). You're still running on sugar, but now it's your own stored sugar, not the meal's.

This is also where hunger shows up, and it's worth knowing that the hunger isn't a fuel gauge. Most of it is ghrelin, a stomach hormone trained to fire at your usual mealtimes; it arrives in waves that crest and recede within roughly 15–45 minutes if you don't eat (Cummings 2001). The full story is in the ghrelin wave.

Hours 12–16: when does fat burning (lipolysis) start?

Somewhere around hour 10–12, liver glycogen runs low and the body reaches for its fat stores. Hormone-sensitive lipase pulls triglycerides out of adipose tissue and breaks them into free fatty acids, which enter the bloodstream and head for the liver (Cahill 2006). This is lipolysis — the start of real fat-burning.

There's a catch: the brain can't use fatty acids directly, because they don't cross the blood-brain barrier. So fat alone doesn't yet feed your head — the body needs one more conversion step, which is what the next phase is for. (This is also the window where the "does X break my fast?" question matters most; our decision tree on what breaks a fast sorts it out.)

Hours 16–24: when do you enter ketosis?

Ketosis is the phase most daily fasters aim for, and it's usually reached around hour 16. The liver starts converting fatty acids into ketone bodies — chiefly β-hydroxybutyrate — which can cross the blood-brain barrier and become fuel for neurons (Owen 1967; Newman & Verdin 2017). A blood β-hydroxybutyrate reading above about 0.5 mmol/L is the clinical threshold. On a typical 16:8 schedule you only touch the edge of it; deeper ketosis takes 18–24 continuous hours. People often notice a quieter appetite and clearer head here. The mechanism, step by step, is in how the body enters ketosis.

Want to know exactly how many hours you have left before this phase? Try the fasting phase calculator.

Hours 24–48: when does autophagy begin?

Autophagy — the cell's recycling program, where damaged proteins and worn-out organelles are broken down for reuse — is driven by sustained suppression of mTOR and insulin (Mizushima & Komatsu 2011). It begins to pick up earlier, in the lipolysis-to-ketosis window, but it rises more sharply after about hour 24 and is the defining process of the one-to-two-day fast (Mattson 2018). It is a gradient, not a switch you flip at a single hour, and the human clinical evidence is more preliminary than the mechanism — so it's worth being measured about it. The gentle, honest version is in autophagy, explained.

Hours 48–72: what happens in a deep fast?

Past 48 hours you're in the deep fast. Growth hormone rises substantially over baseline, which helps preserve lean mass against the catabolic pressure of going without food (Ho 1988). Ketones now supply the majority of the brain's energy rather than just a share of it (Cahill 2006; Owen 1967), and autophagy is at its most active. Beyond 72 hours the marginal return on cellular cleanup falls while the physiological cost keeps climbing, which is why few people should go past three days without medical supervision. How you come out matters as much as how far you go — see how to break a long fast.

How far into the timeline does your fast reach?

The phase you reach is decided entirely by how long you fast — which is why the timeline and your protocol are really the same question. Daily schedules (16:8, 18:6, 20:4, OMAD) live mostly in the lipolysis-to-ketosis band and restart before autophagy gets going. Extended fasts (24, 36, 48, 72 hours) are the only way to spend real time in autophagy and the deep fast. For which protocol reaches which phase, see intermittent fasting schedules.

If you'd rather not do the arithmetic, Intermittent shows you which of these phases you're in right now — live on your home screen, free on Android and iOS — instead of just counting hours.

Frequently asked questions

When will I reach ketosis when fasting?

For most people, ketosis begins around 16 hours into a fast, when liver glycogen has run low enough for the liver to start producing ketone bodies. A 16:8 schedule reaches the edge of it; deeper, steadier ketosis takes roughly 18–24 continuous hours (Cahill 2006; Newman & Verdin 2017).

How many hours of fasting until autophagy?

Autophagy starts picking up in the 12–18 hour window but rises more sharply after about 24 hours, which is why a fast of at least a day is usually cited for it. It peaks during sustained fasts of roughly 36–72 hours (Mizushima & Komatsu 2011; Mattson 2018).

Is the fasting timeline the same for everyone?

The order of the phases is the same for everyone, but the exact hours shift with your last meal, activity level, and individual metabolism. Treat the timings as a well-supported approximation, not a precise clock.

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. Mattson MP, Moehl K, Ghena N, Schmaedick M, Cheng A. "Intermittent metabolic switching, neuroplasticity and brain health." Nat Rev Neurosci, 2018. doi:10.1038/nrn.2017.156
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. 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
9. Related reading: what is intermittent fasting, intermittent fasting schedules, how the body enters ketosis, autophagy explained, the ghrelin wave, and how to break a long fast.