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Burn tells you what the hours mean.

Four things Burn does with your fast, each one with the screens that back it up.

Download free on the App Store See the four reasons

01

It names the stage you are in

02

It helps at the moment you want to quit

03

It shows what your history adds up to

04

It tells you how firm the science is

01

Every hour has a name

Burn tells you when you have crossed from Glycogen Depletion into Fat Mobilisation, what that means, and what usually comes next.

Nine stages, from Fed State to Deep Autophagy, each with its own color, its own detail sheet, and its own note on how firm the evidence is. Open-ended fasts keep going past the last one.

See all nine stages →
Burn's timer at 8 hours 41 minutes, stage Glycogen Depletion Burn's Fat Mobilisation stage detail, covering 14 to 18 hours

The nine stages, on one scale

0-3hFed State
3-8hEarly Fasting
8-14hGlycogen Depletion
14-18hFat Mobilisation
18-24hKetosis Entry
24-48hFat Burning
48-72hDeep Ketosis
72-96hAutophagy
96-168hDeep Autophagy
Burn's struggle support sheet listing six tools, with the current fast at 15 hours 26 minutes Burn's home screen with a hunger timer counting down from 19 minutes 38 seconds

02

Help is always one tap away

Hunger comes in waves that peak and pass. Wait out the wave and it goes. That is easy to read and hard to do at seven in the morning with four hours left.

Whatever hour you are in, Burn keeps six tools one tap from the timer: a hunger timer that runs on your home screen, water first, a symptom check that will tell you to stop, and the reason you wrote down when you started.

6

tools, one tap away

20:00

the wave you outlast

0

food photos to log

See how the support tools work →

03

Your record turns into something you can read

Two weeks in, Burn tells you which day you fast longest, whether the trend is real or noise, and what to do with that.

Insights

Patterns written as sentences, with the arithmetic shown rather than hidden behind a score.

Burn's Insights tab showing a morning brief, new insights, and stat tiles

Journal

Every fast by day, week, and month, with energy, mood, water, and weight alongside the hours.

Burn's Journal tab with the Days view selected, showing an August 2026 calendar of logged fasts, today's ring reading 44, a fast in progress at 1d 2h, and a day total of 1d 18h

Morning Brief

A short daily read built from your own numbers, with its citations attached.

Burn's Morning Brief with today's summary, recovery and readiness, and fasting performance

Where a figure needs sleep or heart-rate data you have not connected, Burn says so instead of estimating. See Morning Brief →

04

Precise about the clock, honest about the biology

Burn maps elapsed time to named stages, and says which of those stages rest on firm evidence and which are active research areas. You get both views.

Well established

Insulin falling, glycogen draining, and fat release are well described in the literature, and the early stages rest on that.

Measurable with a test

Ketones can be measured with a blood or breath test. Burn tells you the stage the clock has reached, not a ketone number.

Active research

Autophagy timing in humans is an active research area, and our page on it says exactly that rather than promising an hour.

Every science claim on this site is drawn from peer-reviewed research and traced to a dated evidence packet. Read the science section →

Side by side

A fasting timer, and a fasting timer that explains itself

A plain fasting timer counts the hours you asked for. Everything below is what Burn adds on top of that count.

A COUNTDOWN

A plain fasting timer

A CLOCK WITH CONTEXT

Burn

While the fast runs

Time remaining against the goal you set.

Elapsed time, the named stage that time has reached, and a rail marking every boundary still ahead.

At the hard hour

The same number, still counting.

Six tools one tap from the timer, including a hunger timer, a hydration prompt and a symptom check.

When you finish

The fast is saved as a duration.

The duration, the stage it reached, an effort record and a place in your streak and personal records.

Over a month

A list of past fasts.

Insights written as sentences, a day, week and month journal, and twelve personal records with the dates they were set.

On the science

Not addressed.

Every stage carries a note on how firm the evidence is, from well established to active research.

What you log

The start and end of each fast.

The same, plus optional water, electrolytes, weight, energy and mood. No calorie database and no food photos.

Where it runs

A phone app.

Native iPhone, iPad and Apple Watch, with widgets, Live Activities, Apple Health and Siri commands.

Your record

Held in the app.

Stored on your device, synchronized through iCloud if you enable it, and exportable as CSV at any time.

The left column describes what a countdown does by definition, not any particular product. See everything in Burn →

Also worth knowing

Four smaller reasons that add up

Nothing to log but the clock

No calorie database, no photographing dinner, no macro grid. One question: when did you stop eating?

Native to Apple, end to end

Native iPhone, iPad, and Apple Watch, with widgets, Live Activities, Apple Health, and five Siri commands.

A plan for wherever you are

Twenty-one guided programs, seven quick plans from 11:13 to alternate-day, and three custom protocols of your own.

Your record stays yours

Apple Health is optional, CSV export is built in, and nothing is collected that the app does not use.

Judge it on one fast

Start the timer tonight and Burn will name every stage you cross between dinner and lunch tomorrow.

Download free on the App Store

iPhone, iPad, and Apple Watch. Requires iOS 18 or watchOS 11.