20 hour fast stages (20:4): what happens hour by hour

The stages of a 20-hour fast, hour by hour: what the body typically does from the last meal to hour 20, how it differs from a 16-hour fast, and who should take care.

In brief: a 20-hour fast, as in the 20:4 schedule, typically passes through the fed state, early fasting and lipolysis, then spends roughly the last four to eight hours in ketosis. Insulin falls from around hour 4, fat release begins around hour 12, and ketone levels keep rising through hours 16 to 20. Autophagy is only beginning to be a factor. These hours are typical ranges, not thresholds, and a fast this long is for experienced fasters.

Fasting for 20 hours and eating within 4 is the 20:4 pattern, sometimes called the Warrior diet in its classical form. It sits between the standard 16:8 and a full 24-hour fast. Where a 16 hour fast only touches ketosis, a 20-hour fast spends a meaningful stretch in it. This page walks the clock; the full 0 to 72 hour picture is in the fasting timeline.

What are the stages of a 20 hour fast?

A 20-hour fast spends time in the fed state, early fasting, lipolysis and ketosis. It stops before the clearer autophagy window that begins around hour 24.

Hour of the fast	Stage	What typically happens	Evidence in humans
0–4	Fed state	Insulin is elevated; glucose 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 to hold blood glucose steady, and by hours 8 to 12 that glycogen is substantially drawn down (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
16–18	Early ketosis	Blood β-hydroxybutyrate is typically around the 0.5 mmol/L threshold; ketones are now a usable brain fuel (Owen 1967; Newman & Verdin 2017)	Well established; timing varies
18–20	Ketosis	An 18-hour fast is described as enough to trigger the switch to ketone-based energy; levels keep climbing through the end of the fast (de Cabo & Mattson 2019)	Well supported; exact levels vary widely

How does hour 20 differ from hour 16?

The main change is depth, not a new phase. Between hours 16 and 20 the body is already in ketosis, and ketone levels continue to rise rather than arrive for the first time. The ketosis mechanism article explains why: with insulin low and liver glycogen depleted, the liver turns more fatty acids into β-hydroxybutyrate, which crosses the blood-brain barrier and fuels neurons (Owen 1967; Newman & Verdin 2017). People often describe a quieter appetite and clearer head in this window, though that is self-report rather than a measured guarantee.

Does a 20 hour fast trigger autophagy?

Only at the very beginning, if at all. 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 20-hour fast sits just short of that point, and autophagy cannot be measured directly in a living person without a tissue biopsy. For the honest version of the evidence, see autophagy, explained.

What does a 20 hour fast look like on the clock?

If the last meal ends at 8 pm, hour 12 is 8 am, hour 16 is noon, and hour 20 is 4 pm, when the 4-hour eating window opens; it closes at 8 pm. Most people compress that window into one large meal and a lighter second one; meeting protein and micronutrient needs in four hours takes planning. The schedules guide suggests 20:4 for experienced fasters, typically three to five times a week rather than every day.

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. Electrolytes matter more as insulin stays low, because sodium handling changes; the electrolyte handbook covers it. Faintness, confusion or palpitations are reasons to stop and eat. A dull headache is more often a sodium issue, which the electrolyte handbook also covers.

Who should be careful with a 20 hour fast?

Treat 20:4 as a step up from 16:8, not a starting point. 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.

Frequently asked questions

What happens during a 20 hour fast?

Over 20 hours the body moves from the fed state through early fasting and lipolysis into ketosis. Insulin falls from around hour 4, fat release begins around hour 12, and ketone levels rise through hours 16 to 20. Autophagy is only beginning to be a factor. Exact hours vary by person (Cahill 2006; de Cabo & Mattson 2019).

Are you in ketosis after 20 hours of fasting?

For most people, yes. Ketone production usually begins around hour 16, so by hour 20 the body has spent several hours in ketosis and ketone levels are higher than at the end of a 16-hour fast, though individual levels vary widely (Newman & Verdin 2017).

Is a 20 hour fast better than a 16 hour fast?

It goes deeper into ketosis but is also harder to sustain and not suited to everyone. The 16:8 schedule is the standard daily protocol; 20:4 is usually for experienced fasters, a few times a week. Better depends on your health, goals 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. de Cabo R, Mattson MP. "Effects of intermittent fasting on health, aging, and disease." N Engl J Med, 2019. doi:10.1056/NEJMra1905136
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. Mizushima N, Komatsu M. "Autophagy: renovation of cells and tissues." Cell, 2011. doi:10.1016/j.cell.2011.10.026
8. Related reading: the fasting timeline by hour, 16 hour fast stages, 72 hour fast stages, intermittent fasting schedules, how the body enters ketosis, what to drink while fasting, and electrolytes while fasting.