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A timer that tells you where you are

Elapsed time to the second, the stage that time has reached, and a rail marking every boundary still ahead. One screen, and it changes as the fast goes on.

The clock counts up, and Burn names what the count means.

Burn's timer at 5 hours 8 minutes on a 16:8 fast, the stage pill reading EARLY FASTING

Burn's timer at 14 hours 36 minutes, the stage pill reading FAT RELEASE

Burn's timer at 18 hours 47 minutes, the stage pill reading KETOSIS

What is on the screen

Four things, and nothing else

No calorie field, no food log, no macro ring. A fasting timer needs the clock, the stage, the progress and the controls.

THE CLOCK

Hours, minutes, seconds

Elapsed rather than remaining, so the number goes up as you get further in. Past 24 hours it reads in days.

THE STAGE PILL

Where that time puts you

One of nine named stages, colored to match. Tap it for the full description of what the research supports at that point.

THE STAGE RAIL

Every boundary ahead

Marked at 3, 8, 14, 18 and 24 hours, filling as you pass each one. You can see the next line coming rather than being surprised by it.

THE CONTROLS

Start, edit, end

Set the plan and start time when you begin, correct the start time if you forgot to open the app, and end the fast whenever you decide to.

As the fast goes on

The screen changes color as you cross each line

Each stage has its own color, from the grey of Fed State through the blues and purples of the first day into the ambers of Fat Burning. Glancing at your phone tells you roughly where you are before you have read anything.

A buzz on your wrist at every boundary

Stage notifications fire as you reach each one, so you find out you have entered Ketosis Entry without checking. Rate-limited, and aware of your quiet hours.

Goal met, then keep going

Passing your target does not stop the clock. The timer marks the goal as met and keeps counting, because a 16:8 that ran to 17 hours is worth recording as 17.

The same fast, on your wrist

The same clock and stage on your wrist, as a complication and a full app, so the timer is readable without taking your phone out.

How the stages work → Burn on Apple Watch →

Burn's timer at 16 hours 22 minutes with the 16:8 goal met and the clock still running

Burn's timer at 1 day 2 hours 33 minutes, the stage pill reading FAT BURNING

The nine boundaries

What the rail is marking

Hover any band to isolate it.

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

The first day runs at one scale and everything after 24 hours is compressed against it at three to one, so the early hours stay readable. Nothing in the biology switches on at a stated hour; Burn draws each line because a timer has to draw one.

Burn's fast screen scrolled down, showing the Morning Brief card, energy check-ins and support tools beneath the timer

Below the clock

Everything the fast needs, on the same screen

Scroll down from the timer and the rest of the fast is there: your Morning Brief, energy check-ins, and the support tools for the hour that tests you. Nothing needs a different tab.

Check in on how you feel

Log energy and mood as the fast runs, so the record holds more than a duration when you look back at which weeks went well.

Support tools, one tap away

Six tools including a hunger timer, a hydration prompt and a symptom check, reachable from the fast screen rather than from a menu.

Program and challenge controls

If the fast is part of a program or a challenge, its progress sits under the timer too, so one screen covers the day.

The six support tools → Morning Brief →

Precise about the clock, honest about the biology

Elapsed time is the one thing a phone can know exactly, so the stages run on it. Onset of each physiological shift varies with your last meal, your activity, your sleep and your metabolism. Burn maps elapsed time to named stages and does not measure ketosis, autophagy, or metabolic state.

Every fast lands in the record

Whatever length it ran to, the fast is saved with the stage it reached and counts toward completions, streaks and twelve personal records. Stop and eat if you feel unwell, whatever the timer says.

Start the clock tonight

Set a plan, start the fast, and Burn names every stage you reach with a buzz at each boundary.

Download free on the App Store

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