16 hour fast stages: what happens hour by hour

What typically happens during a 16-hour fast, hour by hour: from the fed state through lipolysis to the edge of ketosis, and what the evidence does and does not support at hour 16.

The stages of a 16-hour fast, hour by hour: where insulin, glycogen, fat release and ketones typically stand at each point, what the evidence supports at hour 16, and what it does not.

In brief: in a 16-hour fast the body typically moves from the fed state, through early fasting, into lipolysis, and arrives at the edge of ketosis in the last hour or two. Insulin falls from around hour 4, liver glycogen is largely drawn down by hours 12 to 16, and blood ketones are usually still low, around 0.3–0.6 mmol/L, when the fast ends. Autophagy is not meaningfully established at this length. These hours are typical ranges, not thresholds.

A 16-hour fast is the daily fast behind the 16:8 schedule: fast for 16 hours, eat within 8. It is the first common daily protocol that reaches the end of the lipolysis phase and touches ketosis, which is why it is the usual starting point once 12:12 and 14:10 feel easy. This page walks the clock hour by hour. For the full 0 to 72 hour picture, see the fasting timeline.

What are the stages of a 16 hour fast?

A 16-hour fast spends time in four of the six phases: the fed state, early fasting, lipolysis, and the very start of ketosis. It does not reach autophagy or the deep fast.

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
4–8	Early fasting	Insulin begins to fall, glucagon rises, and the liver starts releasing its own glycogen to hold blood glucose steady (Cahill 2006; Anton 2018)	Well established
8–12	Glycogen drawdown	Liver glycogen is drawn down substantially; free fatty acids begin to rise; ketones are only trace (de Cabo & Mattson 2019)	Well established
12–14	Lipolysis begins	Fat cells release free fatty acids into the blood; real fat burning starts (Cahill 2006)	Well established
14–16	Lipolysis to early ketosis	The liver begins converting fatty acids into ketones, mainly β-hydroxybutyrate; levels are typically near the edge of the 0.5 mmol/L threshold by the end (Newman & Verdin 2017)	Ketones: well established. Exact timing varies by person

Why does hunger come and go during a 16 hour fast?

Hunger during a fast is mostly not a fuel gauge. Ghrelin, a stomach hormone, is trained to rise at your usual mealtimes, in waves that crest and fade within roughly 15 to 45 minutes if you do not eat (Cummings 2001). If a 16-hour fast ends at noon, expect a ghrelin wave near your old breakfast time that passes on its own; see the ghrelin wave for how it behaves.

Is 16 hours long enough for ketosis?

At the edge, yes; fully, usually not. The liver starts producing ketones as liver glycogen runs low, typically in the last couple of hours, around hours 14 to 16, but blood β-hydroxybutyrate is commonly only about 0.3–0.6 mmol/L by the end of a 16-hour fast. Steadier, deeper ketosis generally takes 18 to 24 continuous hours (Newman & Verdin 2017; Cahill 2006). The mechanism step by step is in how the body enters ketosis.

Is autophagy active at 16 hours?

Not in a way that can be shown in people. Autophagy is driven by sustained suppression of mTOR and insulin, and its clearest signal in animal studies of liver and brain comes at about 24 hours (Alirezaei 2010; Mizushima & Komatsu 2011). At 16 hours it is, at most, an early foothill, and it cannot be measured directly in a living person without a tissue biopsy. Treat claims that a 16-hour fast delivers autophagy with caution; the honest version is in autophagy, explained.

What does a 16 hour fast look like on the clock?

If the last bite is at 8 pm, hour 4 is midnight (insulin starting to fall), hour 12 is 8 am (glycogen low, fat release starting), and hour 16 is noon (the edge of ketosis). Shifting the same fast to start at 7 pm ends it at 11 am. Because the boundaries are ranges, the exact feel differs from person to person and from day to day.

What is the difference between a 16 hour and a 20 hour fast?

The extra four hours are spent in ketosis rather than at its edge: ketone levels keep rising and the metabolic shift is more complete, but the fast is also harder to live with. The 20 hour fast stages page covers hours 16 to 20 in the same format.

Who should be careful with a 16 hour fast?

A 16-hour daily fast is well tolerated by most healthy adults, but it is not for everyone. Do not begin intermittent fasting at any schedule 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 on medication, a history of gallstones, thyroid, kidney or adrenal conditions, or are over 65. This page is educational and not medical advice; a 16-hour fast can be broken with a normal meal, and you should stop and eat if you feel faint or unwell.

Frequently asked questions

What happens during a 16 hour fast?

Over 16 hours the body moves from the fed state through early fasting and lipolysis to the edge of ketosis. Insulin falls from around hour 4, liver glycogen is largely drawn down by hours 12 to 16, and the liver begins making ketones near the end. Exact hours vary with the last meal and individual metabolism (Cahill 2006).

Do you burn fat during a 16 hour fast?

Fat release (lipolysis) typically gets going around hour 12, once liver glycogen runs low (free fatty acids begin to creep up a little earlier), so the second half of a 16-hour fast is spent mobilising fat. The extent depends on your last meal, activity and metabolism (Cahill 2006).

Are you in ketosis after 16 hours of fasting?

Usually only at the edge. Ketone production begins around hours 14 to 16 for most people, but blood levels are often still below the 0.5 mmol/L threshold at that point; deeper ketosis takes roughly 18 to 24 continuous hours (Newman & Verdin 2017).

Sources

Cahill GF Jr. "Fuel metabolism in starvation." Annu Rev Nutr, 2006. doi:10.1146/annurev.nutr.26.061505.111258
Newman JC, Verdin E. "β-hydroxybutyrate: a signaling metabolite." Annu Rev Nutr, 2017. doi:10.1146/annurev-nutr-071816-064916
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
de Cabo R, Mattson MP. "Effects of intermittent fasting on health, aging, and disease." N Engl J Med, 2019. doi:10.1056/NEJMra1905136
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
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
Mizushima N, Komatsu M. "Autophagy: renovation of cells and tissues." Cell, 2011. doi:10.1016/j.cell.2011.10.026
Related reading: the fasting timeline by hour, 20 hour fast stages, 72 hour fast stages, intermittent fasting schedules, how the body enters ketosis, the ghrelin wave, and what breaks a fast.