Extended fasts · 48-72h

Your first 72-hour fast, guided and done right

Know what to expect hour by hour, when to take electrolytes and how to break your fast without paying for it. Finish light, clear-headed and proud.

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72hFull challenge

48hWarm-up step

6 phasesAlert at each one

Free Data in the EU iOS · Android

Sound familiar?

I want to do a long fast but I'm scared of messing it up

Surprising mental clarity

I tried and quit on day two with a splitting headache

A deep reset

I don't know how to break it without a stomach ache

Rethink your hunger

What you will notice

What you get out of it

This is what changes in your day to day when you give your body well-guided hours without food.

Surprising mental clarity

From day two on, many people describe their head as unusually clear.

Why:In deep ketosis the brain runs on ketones instead of glucose.

A deep reset

You finish feeling light, with far less puffiness in your body.

Why:In long fasts autophagy ramps up: cells recycle what's damaged.

Rethink your hunger

You learn hunger comes in waves. After this you eat calmer.

Why:The hunger hormone rises in waves and drops on its own from day 2.

You do it safely

You know what's normal, what isn't and when to stop. No heroics.

Why:Electrolytes and clear warning signs prevent most problems.

Pride and self-control

Finishing a 72-hour fast changes how you see your own willpower.

Why:Beating a hard challenge builds confidence for new habits.

Exit without an upset stomach

You break the fast step by step and enjoy it instead of paying for it.

Why:After days without food, your gut needs to restart with gentle meals.

SCIENCE · PHASE BY PHASE

What happens in your body, hour by hour

The app tells you the moment you enter each phase. No more waiting around to "see if it works."

12h

Digestion, insulin drops

You run on your last meal

0-12hDigestion, insulin drops

You run on your last meal

Feels like a normal day. Good dinner first, with protein and veggies.

12-24hFat burning

Your body taps stored fat

Hunger waves at your usual mealtimes. Water, salt and black coffee.

24-36hKetosis

Energy coming from fat

A headache or weakness can show up: electrolytes. Keep activity light.

36-48hDeep ketosis

Clear mind, low hunger

Many people hit the sweet spot here: calm and focus.

48-60hPeak autophagy

Cellular cleanup at its peak

Physical energy runs lower. Rest, walk, no workouts.

60-72hHome stretch

Challenge complete

Plan your exit: broth, an egg or yogurt, then wait before eating more.

Your progress

What to expect, week by week

Week 1

Prep with 16:8 and one 24-hour fast to learn how you respond.

Weeks 2-3

First 36-48 hour fast. You dial in electrolytes and routine.

Month 1

Your first full 72 hours, with phase alerts and a clean exit.

Month 3

If it agrees with you, an occasional long fast with 16:8 as your base.

What the science says72-Hour Fast, Guided Hour by Hour

Reviewed on 12/10/2026

Extended fasts of 48-72 hours push the body to run on fat and ketones for fuel, and in supervised clinical settings they are usually tolerated with adverse effects that are mostly mild. Long-term benefits in healthy people are not proven, and they carry risks such as low blood sugar and electrolyte imbalances. Avoid them with diabetes, pregnancy, low body weight or an eating disorder.

Across 768 supervised water-only fasts (≥2 days), 75% of adverse effects were mild and there were 2 serious adverse events. [1]
Among 1422 people on a 4- to 21-day Buchinger fast (200-250 kcal/day) in a clinic, adverse effects occurred in under 1%. [2]
The metabolic switch from glucose to fat and ketones usually happens beyond 12 hours without food, as liver glycogen runs out. [3]

Frequently asked questions

How do I do a 72-hour fast safely?

Come prepared: weeks of 16:8 and at least one 24-hour fast. Drink water, take electrolytes (sodium, potassium, magnesium), skip intense exercise, and stop if you feel strong dizziness, heart palpitations or confusion.

Who shouldn't do a 72-hour fast?

Don't do it if you're pregnant or breastfeeding, or if you have diabetes, are underweight, have heart or kidney problems or gout, take medication, or have had an eating disorder. If you have any doubt at all, talk to your doctor first.

What does it feel like hour by hour?

Through hour 24, hunger comes in waves. Between 24 and 36 hours you may feel weak or get a headache. From 36 to 48 hours many people feel calm and clear. On day 3 physical energy drops: rest and gentle walks.

Which electrolytes should I take?

Sodium (salt in your water), potassium and magnesium are the main ones. Ask a professional about doses, especially if you take medication or your blood pressure runs high or low. Ayúname reminds you when to take them.

How do I break the fast without an upset stomach?

Start small and gentle: broth, eggs or plain yogurt. Wait an hour or two before a normal meal, and skip sugar, fried food and big feasts on day one.

Scientific sources (3)