Intermittent fasting schedules — 12:12 through 72 hours, explained

Every common intermittent fasting schedule, who it's for, and which biological phases it reaches. The 16:8, 18:6, 20:4, and OMAD protocols for daily fasting — plus 24-hour, 36-hour, 48-hour, and 72-hour extended fasts. Plus how to pick one.

Fasting is one dial: how long you go without eating. Everything else — whether you call it 16:8 or "time-restricted eating" or OMAD — is a variation on that one number. What changes as you turn the dial is which biological phases your body passes through, and how hard the fast is to live through.

This is a pillar reference. Each section below names one schedule, states the fasting-to-eating ratio in hours, says which phases the body reaches, and flags the common challenges. The comparison table at the end summarises all twelve in one glance. For the underlying biology — what Ketosis, Autophagy, and the other phases actually are — see what is intermittent fasting and the individual phase articles linked throughout.

The daily protocols

Daily intermittent fasting means repeating the same fast-eat rhythm every day. The window is typically defined by when you stop eating the evening before and when you break the fast the next day. The numbers before and after the colon are fasting hours and eating hours.

12:12 — the gentle baseline

Fast 12 hours, eat within 12 hours. Almost everyone is already doing this if they stop eating by 8pm and have breakfast at 8am. It reaches the end of the Fed state but typically no further. Insulin drops, the body has burned through most of its post-meal glucose, and liver glycogen begins to drawdown. No real metabolic shift yet.

Who it's for: absolute beginners, anyone whose doctor has flagged fasting risk, people using it to stop evening snacking. It's also the ramp stage for a longer-protocol plan.

14:10 — the first taste of fat burning

Fast 14 hours, eat within 10. This is where Early fasting ends and the liver begins to favour fat oxidation. Hunger is mild and transient — the ghrelin wave peaks right around the time you'd normally eat breakfast. Hormonally, insulin is low and glucagon is rising.

Who it's for: people two weeks into a gentle ramp. People who have done 12:12 for a while and want more.

16:8 — the standard daily protocol

Fast 16 hours, eat within 8. The most-searched fasting schedule, for a reason: it's the first daily protocol that reliably reaches Ketosis. Liver glycogen is substantially depleted, fat mobilisation has been under way for four hours, and the liver starts producing β-hydroxybutyrate right around hour 16. Brain fuel begins shifting.

Practically, 16:8 means something like stopping eating at 8pm and not eating again until noon the next day. For most people, it fits normal life — skip breakfast, eat lunch and dinner.

Who it's for: the default daily IF protocol. If you are picking one and have no reason to pick another, pick this one.

18:6 — reliable ketosis

Fast 18 hours, eat within 6. Two extra fasting hours past 16:8, which keeps you comfortably inside Ketosis for the last two hours instead of arriving at its edge. Ketone levels are higher; the metabolic shift is more complete.

The eating window becomes a narrow lunch + dinner (roughly 2pm–8pm) or an early dinner-only pattern. Some people find this easier than 16:8 because the eating window carries social meals.

Who it's for: people who have done 16:8 for 4–6 weeks and want more. People optimising for cognitive focus — the ketone-fueled brain state is more present at 18h than 16h.

20:4 — the Warrior

Fast 20 hours, eat within 4. Also called the Warrior diet in its classical form. Deep into Ketosis — ketone levels continue to rise through the fast. The eating window is tight enough that most people naturally compress to one large meal plus a light second meal.

Who it's for: experienced fasters, people who've done 16:8 or 18:6 for a few months. People who find small-meal rhythms harder than concentrated ones.

OMAD — one meal a day

Typically 23:1 — fast 23 hours, eat within one. Functionally equivalent to a daily 23-hour fast. You pass through every phase up to the edge of Autophagy, though you don't actually cross into Autophagy in a true sense since Autophagy begins ~24h and needs sustained fasting to peak.

OMAD is demanding. Meeting daily protein and micronutrient targets in a single meal takes planning. Caloric deficits can stack up silently.

Who it's for: experienced fasters. Not a starting point. Useful for specific social-schedule situations (work travel, single-meal family cultures), less useful as an optimisation.

The extended fasts

Extended fasts run 24 hours or longer. At this point you are well past what daily eating patterns can absorb — extended fasting is its own practice, usually done weekly or monthly rather than daily.

24 hours — the daylong fast

One full day without eating, typically dinner-to-dinner. The first few hours of Autophagy kick in. Electrolyte supplementation (sodium, potassium, magnesium) starts to matter — sodium loss accelerates once insulin has been low for most of a day.

Who it's for: people who do daily 16:8 or 18:6 and want to occasionally go deeper. Weekly 24h fasts are a common rhythm.

36 hours — deep autophagy

Dinner one day, skip all meals the next day, dinner the following. Sustained Autophagy — mTOR is suppressed for long enough that lysosomal recycling is substantively active. Ketone levels are high and stable. Mental clarity is often reported around the 30–36h mark.

Hunger is paradoxically less of a problem at 36h than at 20h — the ghrelin cycle has flattened and appetite has largely switched off. The challenge shifts from hunger to energy management and electrolytes.

Who it's for: people with a few successful 24h fasts behind them. Monthly or bi-weekly rhythms are typical.

48 hours — the peak autophagy window

Two full days without food. Autophagy is at its most active; growth hormone has begun to climb (preserving lean mass against the catabolic pressure); ketones are supplying 40–60% of brain energy.

Demanding. Electrolytes are non-negotiable at this depth, not optional. Physical performance drops noticeably. Sleep can be disrupted (elevated cortisol) or improved (deep sleep phase), person-dependent.

Who it's for: experienced extended fasters. Usually done quarterly rather than monthly, often with a specific goal (immune-system reset, surgery prep with physician supervision, research protocol compliance).

72 hours — the prolonged fast

Three days without food. Growth hormone has risen 5–10× baseline. Ketones now supply ~60–70% of brain energy. Autophagy is saturating — extending past 72h produces diminishing marginal returns on cellular cleanup while exponentially increasing risk.

72 hours should only be attempted by people who have done multiple 48h fasts, have no medical contraindications, and have read our refeed protocol. Breaking a 72h fast incorrectly can cause meaningful short-term metabolic stress (hypophosphatemia risk in edge cases). See how to break a long fast before you start one.

Which schedule should I start with?

If you have never fasted intentionally: start at 12:12 for one week, then 14:10 for a week, then 16:8. Three weeks and you are at the standard daily protocol, having reached the edge of Ketosis gently. That is the ramp the app's Plan Creator builds for you.

If you've done short IF before and want to deepen: pick 16:8 for baseline, then add one 18:6 day per week, expand over a month.

If your goal is weight loss: the schedule matters less than the total weekly caloric balance. 16:8 is plenty. Deeper protocols help at the margins but the wins come from what you eat, not how long you fast.

If your goal is autophagy: a weekly 24h fast plus daily 16:8 gets you most of the benefit with far less cost than doing 48h fasts. For peak autophagy, you need sustained extended fasts, but they are not free.

How long does it take to see benefits?

Metabolic benefits — improved insulin sensitivity, better fat oxidation — show up within 2–4 weeks of consistent 16:8. Measurable weight change follows caloric balance, not the protocol per se; expect 2–4 weeks at a sustained deficit to register. Autophagy benefits are harder to observe directly; they compound over months.

Can I do intermittent fasting every day?

Yes for the gentler protocols (12:12, 14:10, 16:8) — they are sustainable indefinitely for most healthy adults. For 18:6 and tighter, most people find 4–6 days per week works better than 7, giving the body one or two days of normal eating for hormone and social-rhythm balance. Extended fasts (24h+) are not daily by design.

Schedule comparison

Schedule	Fast : Eat	Deepest phase reached	Difficulty	Typical cadence
12:12	12 : 12	End of Fed	★	Daily
14:10	14 : 10	Early → Lipolysis edge	★	Daily
16:8	16 : 8	Early Ketosis	★★	Daily
18:6	18 : 6	Mid Ketosis	★★	Daily
20:4	20 : 4	Deep Ketosis	★★★	3–5×/week
OMAD	23 : 1	Ketosis–Autophagy edge	★★★	3–5×/week
24 h	24 : 0	Early Autophagy	★★★	Weekly
36 h	36 : 0	Deep Autophagy	★★★★	Monthly
48 h	48 : 0	Peak Autophagy + GH rise	★★★★	Quarterly
72 h	72 : 0	Prolonged / Deep fast	★★★★★	Occasional, with care

The one rule that matters more than the schedule

What you eat in your eating window matters more than how long your fasting window is. A 20:4 fast followed by ultra-processed snack food will underperform a 14:10 fast followed by whole foods. Intermittent fasting is not a licence to eat anything — it's a rhythm for eating fewer times. The rhythm compounds only if the food is sound.

This is not medical advice. Do not begin extended fasting (24h+) if you are pregnant or breastfeeding, have a history of disordered eating, take insulin or other glucose-lowering medication, or have a chronic condition your doctor has flagged as fasting-sensitive. If you are unsure, talk to your doctor first.

Sources

1. Moro T, Tinsley G, Bianco A, et al. "Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males." J Transl Med, 2016. doi:10.1186/s12967-016-1044-0
2. Tinsley GM, La Bounty PM. "Effects of intermittent fasting on body composition and clinical health markers in humans." Nutr Rev, 2015. doi:10.1093/nutrit/nuv041
3. 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
4. Stekovic S, Hofer SJ, Tripolt N, et al. "Alternate day fasting improves physiological and molecular markers of aging in healthy, non-obese humans." Cell Metab, 2019. doi:10.1016/j.cmet.2019.07.016
5. 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
6. Horne BD, Muhlestein JB, Anderson JL. "Health effects of intermittent fasting: hormesis or harm? A systematic review." Am J Clin Nutr, 2015. doi:10.3945/ajcn.115.109553
7. Related reading: what is intermittent fasting, how the body enters ketosis, autophagy explained, drinks while fasting, the electrolytes guide, and how to break a long fast.