Intermittent fasting, tracked from what you actually ate

The problem with a fasting timer

A fasting app is usually a stopwatch. You press start when you finish dinner, you press stop when you eat again, and the number in between is your fast. That works right up until the evening you forget to press it — and then the app is confidently counting a fast that ended four hours ago, or giving you nothing for one you actually did.

slotho takes the other route. It already knows what you ate and when, because you logged it there. The fast is worked out from that.

The schedule is a plan. A logged meal is the truth.

You set an eating window — the hours you intend to eat in. That is the plan, and it is what the reminders and the stage timings are built from.

The moment you log food, though, the plan gives way to the record. Eat at 22:30 after a window that closed at 20:00 and the fast restarts at 22:30, rather than claiming two and a half hours that never happened. Finish at 17:00 in a window that ran until 20:00 and you are credited with the three hours the schedule would have ignored. It corrects in both directions, which is the half most timers get wrong.

Not everything counts as breaking a fast. Anything above 50 kcal does, by default — high enough that a black coffee or a herbal tea does not, low enough that any real food does. The threshold is yours to move, down to zero if you want the strict reading.

And there is a floor. A meal only counts as evidence about the fast in progress if it falls inside the eating window that has just closed. Someone who stopped logging three days ago has not been fasting for three days, and slotho will not tell them they have — with nothing logged in the relevant span it falls back to the scheduled window and behaves exactly as it did before.

Your eating window, day by day

Set First meal and Last meal by, and underneath it tells you what you have actually built: 16 h fasting · 8 h eating. It lives in Settings › Fasting.

Every weekday carries its own window. Most people set one and apply it to all seven — but if your Saturday looks nothing like your Tuesday, it does not have to.

The six stages, hour by hour

As a fast runs, slotho marks six commonly cited milestones. Each row shows the hour it begins and the clock time it will fall at for the window you have set, and whichever stage you are in right now is highlighted.

0 h · Digestion — your body is busy processing the last meal; blood sugar and insulin are at their highest.
4 h · Blood sugar falls — blood sugar and insulin ease back down as the last meal is absorbed.
8 h · Glycogen burning — your body draws on glycogen, the carbohydrate stored in the liver and muscles.
12 h · Fat burning — with glycogen running low, your body shifts towards burning stored fat for fuel.
16 h · Ketosis — fat is increasingly converted into ketones, an alternative fuel for body and brain.
24 h · Autophagy — longer fasts are thought to encourage the body’s cellular clean-up and recycling processes.

Those onset hours are the approximate boundaries usually cited for them. They vary from person to person, and they are educational — a way of reading the clock, not a measurement of you.

The stages your window will never reach

A 16:8 does not reach autophagy. It cannot: the fast ends eight hours before that milestone would arrive. slotho draws the unreachable rows greyed back rather than dangling them in front of you, because a stage list that implies you are working towards something your own schedule rules out is just a number going up.

Why the hours move

The onsets above are a shared reference, not a timetable your body agreed to. Four things shift them, and they shift them a lot:

What your last meal was. A large, carbohydrate-heavy meal tops up glycogen and pushes everything later. A small, low-carbohydrate one pulls it earlier.
What you have done since. A hard training session burns through glycogen quickly; a day at a desk does not.
How you normally eat. Someone habitually low-carbohydrate is metabolically closer to ketosis before the fast even starts.
You, specifically. Body composition, sleep, age and ordinary metabolic variation all move the numbers, and none of them are in the table.

Two of the six are worth naming individually. Fat burning is a gradual change in proportions rather than a switch that flips — you were burning some fat before hour twelve, and you will still be using some glucose after it. And ketosis at sixteen hours is the figure everyone quotes and the shakiest of the six: someone who ate a large plate of pasta at 8pm and someone who ate grilled fish and salad are not going to arrive at the same place by noon.

Autophagy is the one anyone can say least about. Much of what gets repeated confidently online comes from cell and animal studies, where it can be measured directly; measuring it in a living human being is considerably harder, and the timing in people is not settled science. If you find a page telling you autophagy begins at hour eighteen, treat the precision as marketing rather than measurement.

The honest summary is that the stages describe a sequence reliably and a schedule only loosely. The order is right. The clock is a rough guide.

So what are the markers good for?

Orientation, mostly. A fast is a long stretch of not much happening, and knowing roughly where you are in it makes the hours easier to sit with. Hour fourteen feels different when you know what is generally going on at hour fourteen, even approximately.

What they are not is a readout. slotho is not measuring your ketones — it has no way to, and neither does any app working from a phone and a watch. It counts the hours since your eating window closed and tells you what is commonly associated with that point. That distinction is written into the source of the stage table itself, which describes the onsets as “commonly cited approximate boundaries — educational copy, deliberately non-medical”.

Reminders, if you want them

All of them are off until you turn them on. You can be told when your eating window opens and closes, and — separately — as each stage begins.

Stages that would land while you are asleep are not fired at three in the morning. They are held back and delivered as a single recap when your day starts: “Your fast moved through: …”. A sixteen-hour overnight fast should not cost you a night’s sleep to observe.

When a logged meal moves the start of the fast, the stage alerts move with it — the ones planned against the old clock are cancelled rather than left to fire at the wrong time.

If you don’t fast

Then say so, and the feature goes away. Setup asks a plain question — do you follow an eating window? — and answering no hides the fasting line on Today, strips fasting out of the widget, and silences both sets of reminders. It stays reachable under Settings › Fasting for whenever that changes.

Worth spelling out, because a fasting timer bolted onto a food app is usually unremovable furniture.

Where it appears

On the Today screen, where a card shows the stage you are in, how long the fast has run and what comes next, and the daily rhythm sets your window beside your sleep. On the home-screen widget, which works out the current stage itself and refreshes when the next one is due. In Settings › Fasting, with the schedule and the full stage timeline.

And on the Apple Watch: the watch’s first page and its complication name the stage of the fast in progress, counted from the same start your iPhone uses.

What it costs

Fasting is free, and not in the sense of a trial. Per-day schedules, the stage timeline and every reminder are in the free app, next to unlimited food, drink and symptom logging and logging on Apple Watch.

PRO adds the Daily Balance score, insights, the weekly recap, your full history and AI with 1,000 credits a month — 1 month of PRO comes free with the app, and current prices are shown on the App Store in your own currency. slotho itself is a free download for iPhone, iPad and Apple Watch.

Fasting is genuinely not right for everyone. If you are pregnant or breastfeeding, have a history of disordered eating, or manage diabetes or medication that depends on regular meals, that is a conversation for a doctor rather than an app.

slotho supports general wellness and education. It is not a medical device and does not provide medical advice. Talk to a qualified professional before making changes to your diet, fasting or exercise.

Net carbs, and how slotho rates a food for keto covers the other half of what people fast alongside. Can Apple Health track food? explains what is written to your Health store and what is only read from it. The slotho FAQ answers the shorter questions, including the ones about notifications and PRO.