FastWithin: a fasting timer built for real life

I ate

Ate something? Tap I ate.

Most fasting apps treat a snack like a failure. FastWithin treats it like data.

Tap I ate.One big button on your home screen.
Say when.Just now, 15, 30 or 40 minutes ago, an hour ago, or an earlier time. Forgot to log it? Pick "40 minutes ago" and the math fixes itself.
Your fast restarts.It restarts from that exact moment. The earlier fast and every restart stay in your history, with no judgment.

Changed your mind? Undo it within 10 seconds. Want to remember what it was? Add a note, like Greek yogurt.

The I ate screen in FastWithin: a note reading Greek yogurt, the time 40 minutes ago selected, and a Restart my fast button.

A woman in a dim kitchen reaches for the wall switch, her phone resting on the counter. — Lights off in the kitchen.

A woman with silver hair reads a book in bed beside a warm lamp, a cup and her phone on the bedside table. — Then a quiet night.

The Kitchen's closed screen: a moon, the words 7 nights in a row, and a resting 3D body.

Kitchen closes

Kitchen closes before bed.

Set your bedtime (weeknights and weekends can differ) and FastWithin closes the kitchen 2, 3 or 4 hours before bed.

A heads up 30 minutes before, and a calm note at the cutoff.
A "Kitchen closes in 1h 20m" chip on your home screen.
A streak of nights you made it.
Optional auto-start. Your fast can start by itself at the cutoff.
Ate late anyway? It's logged with a kind note and a time to aim for tomorrow.

Why before bed? The app shows what research suggests about sleep, heartburn and overnight blood sugar, with the studies behind it. Some of those studies are small, and results differ from person to person.

Body Forecast

Your next 24 hours within.

Every fasting app shows you a timer. FastWithin shows you the forecast: when the kitchen closes tonight, when your fuel switches, when fat burning begins, when you reach your goal, and when a hunger wave is likely, from the times you usually eat.

Built from the same checked zone math the timer uses, so it stays honest.
Hunger waves come from your own meal log, kept on your phone. Research suggests hunger rises around the times you usually eat, then eases.
Not fasting yet? It shows what happens if you start now.

Times are estimates. Your body, your last meal and your day all move them.

Body Forecast

Your next 24 hours within

☾☾⚑∿

now

Kitchen closes in 30 minutesA heads up before the cutoff.
Kitchen closes3 hours before bed.
Fat burningFat use may climb, early ketones.
BedtimeSleep covers a good stretch of your fast.
CleanupCells may begin to recycle worn-out parts.
16 hour goalEat when you're ready, or keep going.
Hunger wave likelyAround a time you usually eat. It tends to pass. Water helps.

Zones count from the moment your fast began.

Honest body zones

Seven zones, hour by hour.

A cinematic 3D body shows what research suggests is going on inside as you fast. Each zone says what is happening in plain English, why it matters, one practical tip, and how we know.

Swipe the timeline to see what's ahead, or tap any part of the ring to fly to that zone.

The Cleanup zone card in FastWithin: the 3D body with a teal spiral, and the line Research suggests your cells may begin to recycle worn-out parts.

Fed

0 to 4 hours

Blood sugar rises, then falls

Your body is digesting your last meal. Blood sugar and insulin rise after you eat. Then they begin to fall over the next few hours.

Stomach and gut at work, sugar into the blood

Settling

4 to 8 hours

Leaning on stored sugar

Your meal is mostly absorbed. Insulin keeps falling. Your liver releases stored sugar (glycogen) to keep your blood sugar steady.

The liver releases stored sugar

Switching

8 to 12 hours

Stored sugar may run lower

For many people, the sugar stored in the liver runs lower around now. The body starts to pull more energy from fat. The full switch to fat and ketones (a fuel made from fat) often comes later, between 12 and 36 hours.

Stored sugar runs lower, fat steps in

Fat burning

12 to 16 hours

Fat use may climb

For many people, the body now uses more fat for fuel. The liver makes ketones from fat, and ketone levels begin to rise (early ketosis). At first, the rise is small.

Fat in use, the liver makes ketones

Cleanup (autophagy)

16 to 24 hours

Cleanup may begin

Research suggests your cells may begin to recycle worn-out parts. This is called autophagy. Cleanup may begin around this time. But human timing is not proven, and most of the data come from animal studies.

Cells may recycle worn-out parts

Deep

24 to 48 hours

Ketones climb higher

Ketone levels usually keep rising as your body relies more on fat. In studies of healthy men, growth hormone release went up during fasts of two days or more.

Ketones reach the brain

48 hours and up

48+ hours

Talk to a doctor first

Fasts this long can cause dehydration in some people. They can also lower blood minerals called electrolytes. Eating again too fast after a long fast can cause a serious problem called refeeding syndrome.

Rest, water, and a doctor's okay

How we know

Every zone card in the app has a "How we know" note with real sources: NIH pages and peer-reviewed studies. We say "research suggests" and "may" on purpose, we tell you when the evidence comes from animals, and we never promise weight loss or cures. The hours are ranges, not exact times, and the zones overlap.

See the sources behind each zone

Fed 0 to 4 hours

Medical references describe how blood sugar and insulin rise after a meal in healthy adults. Blood sugar then comes back down over several hours, and insulin release drops with it. How fast this happens varies from person to person and meal to meal.

Settling 4 to 8 hours

Medical references call the time from about 4 to 6 hours after a meal the postabsorptive state. Then the liver supplies most of your blood sugar, mostly from stored glycogen. In one study of healthy adults, insulin was much lower at 24 hours than at 4 hours. Timing varies from person to person.

Switching 8 to 12 hours

A review puts the full switch at about 12 to 36 hours, depending on your liver's sugar stores and how active you are. In healthy adults, about half of the sugar released into the blood after an overnight fast was newly made, not taken from storage. So stored sugar is already running lower by then.

Fat burning 12 to 16 hours

In one small study, ketones were a little higher after an 18-hour fast than after a 12-hour fast. In another study of healthy adults, ketones were still low at 4 hours and clearly higher at 24 hours. A review says the switch to fat and ketones usually comes after about 12 hours. Size and timing vary by person.

Cleanup (autophagy) 16 to 24 hours

Most of the evidence comes from animals. In mice, autophagy rose in several organs after about 24 hours without food, but it varied by organ. In one small human study, a blood-cell gene linked to autophagy was higher after an 18-hour fast. That is a clue, not proof. A review says the exact timing of effects like autophagy is still largely unknown.

Deep 24 to 48 hours

Two small studies of healthy men found higher growth hormone release during fasting. It was about 5 times higher on the second day of a fast (9 men) and higher during a 5-day fast (6 men). In another study, ketones were about 4 times higher at 48 hours than at 24 hours.

48 hours and up 48+ hours

In a chart review of 768 medically supervised fasts of 2 days or more, tiredness, nausea, headache, and near-fainting were common. Two people needed hospital care, one for dehydration and one for low blood sodium. Refeeding syndrome is mainly described in people who are malnourished or who went more than 5 days with little or no food. How likely it is after shorter fasts is not well known.

Kitchen closes why stop eating before bed

Research suggests that eating close to bedtime may affect a few things. Here is what studies have found so far. Some of these studies are small, and results can differ from person to person.

Safety notes who should check with a doctor first

The safety text in the app (pregnancy, nursing, under 18, diabetes, medicines, eating disorders, long fasts) rests on these.

Mirror Within

Your zone, on your own body.

Point the camera at yourself. The glass organs appear on you, lit by the zone you're in right now: the liver's stored-sugar gauge, the fat layer warming up, ketones reaching the brain.

Capture and share it. One tap makes a picture with your hours, your zone and the moment.
Runs on your phone. Pose tracking happens on the device. The camera feed is never uploaded.
Works on a photo too. Pick any picture of yourself and see where you were.

No other fasting app does this. It's free in FastWithin.

Mirror Within: a woman on a balcony at sunrise with the glass organs drawn over her body, the liver glowing orange, and the line 13h 42m, Fat burning.

Family CircleCode K7M2PX · 4 of 5

☾ Circle Night 3 of 4 made last night's kitchen cutoff.

Family Circle

Fast together. See where everyone is.

Up to five of you, each with your own ring. One screen shows who's fasting, which zone they're in, and who made the kitchen cutoff last night. A code invites the family. One Family Plus covers all five.

Your ring fills as your hours climb, colored by the zone you're in.
Circle Night. Who made it to bed without a late snack.
Only first names, start times and goals leave the phone. Your history, meals and notes never do.

Family Plus is $79.99 a year for five people. Lifetime includes it for good.

Within Score

One number for the habit.

Goals reached, kitchen nights made, streaks kept. Your Within Score grows with every clean fast and every night the kitchen stayed closed, and milestones land as you go: your first 16 hours, seven kitchen nights in a row, your first full day.

It is a habit score from your last 28 days, built only from what you logged. It is not a health score, and it never claims anything about your body.

Within Score

0/ 1000

A

19 goals22 kitchen nights9 day streak

🏆 First goal reached🏆 First 16 hours🏆 7 kitchen nights in a row🏆 100 hours fasted

A day within

One big button. No setup.

Tap Start fasting now and go. Everything else is there when you want it.

A woman stretches on a small balcony at sunrise, a glass of water on the ledge. — **6:40 AM** Hour 14. Fat burning.

A woman writes in a notebook at a sunlit desk with a glass of water beside her. — **9:30 AM** Hour 17. Cleanup may begin.

A family of four laughs over an early dinner at a wooden kitchen table. — **5:30 PM** Dinner together. Tap I ate.

A man closes the fridge in a dark kitchen, holding a glass of water instead. — **8:15 PM** Kitchen's closed. Water instead.

A quiet kitchen at night with the lights off and moonlight through the window. — **11:00 PM** Lights off. Your fast runs on.

A woman opens bedroom curtains to soft morning light. — **6:30 AM** Hour 13. Sleep did the work.

Goals that fit

Pick 12, 14, 16, 18, 20, 24, 36 or 48 hours.
Notifications you control

Zone changes, your goal, the kitchen cutoff, water breaks and a weekly look back. Each one has its own switch, and they keep working after your phone restarts.
History without judgment

Your longest fast this week, goal streak, kitchen streak and a weekly chart.
Right across time zones

Correct times across time zones and daylight saving changes.
Looks the way you like

Dark mode first, light mode too, large text support, and a 2D body for phones that prefer it.
See what's ahead

Swipe the timeline to preview later zones, or tap any part of the ring to fly to that zone.

Fair pricing

The timer is free for good.

No paywall before your timer. Plus is optional, and cancelling takes one tap.

Free, for good

$0

The timer
I ate, with back-dating and undo
Kitchen closes
Every zone, with its sources
The 3D body

No account. No ads. No paywall before your timer.

FastWithin Plus

Optional. Adds:

AI photo logging. Snap your food and AI names it. The photo is never saved by FastWithin.
A weekly pattern coach built from your own history.
Smart bedtime, full history and charts.
Scheduled fasts, custom goals, share cards and themes.
Family Circle (Family Plus and Lifetime). Up to five people fast together and see each other's rings, zones and kitchen nights on one screen.

Monthly$4.99a month

7-day free trialYearly$29.99a year

Family Plus$79.99a year, 5 people

Lifetime$249.99pay once, includes Family

We remind you 2 days before any trial charge. Manage or cancel in Settings takes you straight to Google Play in one tap.

Private by design

No account. No sign-up.

Your fasting history stays on your phone. FastWithin has no ads and no tracking.

No account

Open the app and start. There is nothing to sign up for.
Stays on your phone

Your fasts, meals, notes, bedtime and settings are stored on the device. Notifications are scheduled on the phone too.
No ads, no tracking

No ads, and no analytics or tracking SDKs in the app. This website uses no cookies.
One optional cloud feature

Photo I ate, in Plus, sends a photo over HTTPS to our AI helper (it uses Google's Gemini) only to name the food. FastWithin does not store it. Skip it and nothing leaves your phone.

Read the privacy policy Delete your data

Questions

Good questions, plain answers.

What does I ate do?

Tap I ate and your fast restarts from that exact moment. Forgot to log it? Pick 40 minutes ago, or an earlier time, and the math fixes itself. Changed your mind? Undo it within 10 seconds. Every fast and every restart stays in your history, with no judgment.

Is FastWithin free?

The core is free for good: the timer, I ate, Kitchen closes, every zone with its sources, and the 3D body. There is no paywall before your timer. FastWithin Plus is optional: $4.99 a month or $29.99 a year with a 7-day free trial. Family Plus is $79.99 a year for up to five people and adds the Family Circle. Lifetime is $249.99 once and includes the Family Circle. We remind you 2 days before any trial charge, and Manage or cancel in Settings takes you straight to Google Play in one tap.

Do I need an account?

No. There is no account and no sign-up. Your fasting history stays on your phone.

Is FastWithin medical advice?

No. FastWithin is a timer and education app, not medical advice. Fasting is not right for everyone. Talk to a doctor first if you are pregnant or nursing, have diabetes, take medication, have a medical condition, or have a history of an eating disorder. FastWithin is for adults 18 and over.

How sure is the science behind the body zones?

The hours are ranges, not exact times, and the zones overlap on purpose. Your timing depends on your last meal, your activity and your body. We say "research suggests" and "may" on purpose, we tell you when the evidence comes from animals, and we never promise weight loss or cures. Cleanup (autophagy) is the least certain zone: most of that research is in animals, and human timing is not proven. Every zone has a "How we know" note with its sources, and you can read them here.

What happens to the photo in Photo I ate?

Photo I ate is optional and part of Plus. The photo is sent over HTTPS to FastWithin's AI helper, which asks Google's Gemini to name the food and sends the name back. FastWithin does not store the photo. Google's service may keep what it receives for a limited time under its own terms, so please don't photograph people or anything private. You can always type what you ate instead. The details are in the privacy policy.

What is Mirror Within?

Mirror Within is a Plus feature. Point your phone camera at yourself and the glass organs from the 3D body appear on your own body, lit by the zone you are in right now. The pose tracking runs on the phone itself, so nothing is uploaded and no video leaves your device. It is a way to see the zones on you, not a scan or a measurement, and it does not tell you anything medical.

What is the Body Forecast?

The Body Forecast is your next 24 hours within, on one timeline: when the app expects your zone to change, your Kitchen closes cutoff, your bedtime, your goal, and the hunger waves you are likely to feel, worked out from your own meal log. It is an estimate, not a promise. The zone times are ranges and your body may run earlier or later, so treat it as a map of what research suggests, not a measurement.

What is the Family Circle and how many people can join?

The Family Circle lets up to five people fast together. Each person gets a ring on the same screen with their live zone, so you can see who is in Fat burning and who just ate. Only the live fast is shared (a first name, start time, goal and body); meals, notes and history never leave anyone's phone. The Family Circle comes with Family Plus ($79.99 a year for up to five people) and with Lifetime ($249.99 once).

Does fasting change by age or gender?

The zone hours are the same for every adult; what research suggests about the body at 12 or 16 hours does not change with age or gender. Who should fast, and for how long, is a different question. Women often do best starting at 12 to 14 hours. Over 65, shorter fasts with plenty of water and protein tend to be the safer place to start. If you take blood sugar or blood pressure medicine, talk to a doctor first and start at 12 to 14 hours. If you are pregnant or nursing, you should not fast. The Health profile in the app (blood sugar, blood pressure, pregnant or nursing, age, woman or man) only changes the goals the app suggests and adds a few check-ins. It is not medical advice.

I forgot to start my fast. What do I do?

Tap I finished eating earlier. Pick 4h ago, 10h ago, Last night, or set a time. The timer counts from that moment and shows which zone you are already in, so a fast that really began at 4 AM can be set at 2 PM and the math fixes itself.

Does FastWithin work when I travel across time zones?

Yes. Every time is stored as an absolute moment and shown in your phone's current time zone, so a fast you started in New York still shows the right hours when you land in London, and daylight saving changes do not add or remove an hour. Kitchen closes follows your new local bedtime.

When can I get FastWithin?

FastWithin is an Android app. It is in closed testing on Google Play now, so the Google Play badge on this page is not a link yet. It will link to the listing when the app launches.

How do I delete my data?

Open Settings and tap Delete all my data, or uninstall the app. There is no account, so there is nothing to delete on our side. The delete data page has the details.

Be first in line

FastWithin is in closed testing on Google Play. Leave your email and we'll send the install link the day it opens. No newsletter, no spam.

See what's happening within.