Hour 0 to 4
Digesting your last meal
Sugar from your meal pours into your blood, mostly as glucose. Insulin tells your liver and muscles to store it as glycogen, a reserve for later. The rest goes into your fat cells.
Your meal
A fasting timer for iPhone
Watch your fast from the inside.
A timer built on a see-through 3D body. Hour by hour, it shows which fuel you’re running on and which organs are doing the work.
Coming soon to the App Store
See the first 72 hours
Hour by hour
The first 72 hours.
Scroll through a fast. This is the same body that sits behind the timer in the app, showing only the organs at work.
Hour 4 to 12
Running on your stores
Your meal is stored and your blood sugar drifts down. Your pancreas sends out glucagon, telling your liver to turn glycogen back into glucose for your brain. In one study, about two thirds of that glucose was being built new from smaller molecules.
Stored sugar
Rothman et al., Science, 1991 (7 adults)
Hour 12 to 16
The switch window opens
Somewhere between hour 12 and hour 36, your liver’s glycogen runs low and your main fuel shifts from glucose to fat. A fuller store pushes the switch later. Moving around brings it sooner.
Sugar, shifting to fat
Anton et al., Obesity, 2018 (review)
Hour 16 to 24
Burning fat, breathing it out
As you shift to fat, your fat cells break their stored fat into fatty acids and your muscles burn them. The carbon leaves as carbon dioxide. About 84% of the weight of fat you burn, you breathe out.
More and more fat
Meerman & Brown, BMJ, 2014
Hour 24 to 36
A full day
In that 1991 study, after the first 22 hours about 82% of the glucose the liver sent out was newly made, not taken from its store. Hunger tends to come in waves around your usual mealtimes, and each wave passes on its own.
Mostly fat
Rothman et al., Science, 1991 (7 adults)
Hour 36 to 48
Ketones rise
Fat pours out of your fat cells faster than your liver can burn it, so it turns the extra into ketones. They flow out to your heart, muscles and nerves as fuel. Some become acetone and leave on your breath.
Fat and ketones
If you have diabetes, ketones can build up until your blood turns acidic (diabetic ketoacidosis), which is dangerous. Talk to your doctor before fasting. American Diabetes Association
Anton et al., Obesity, 2018 (review)Hour 48 to 72
Your brain changes fuel
Your brain has run on glucose your whole life. Now more and more of its energy comes from ketones made from your fat. In patients who fasted for 5 to 6 weeks, ketones replaced glucose as the brain’s main fuel.
Fat, and ketones for the brain
Owen et al., J Clin Invest, 1967 (3 patients)
Hour 72
Three days in
In the 1991 study, by the third day 96% of the liver’s glucose was newly made. Your metabolism hasn’t crashed: in 11 lean adults fasting 84 hours, resting energy use was about 14% higher on day 3.
Fat and ketones
After more than about 5 days with little or no food, eating again too fast can cause refeeding syndrome, which can stop the heart. Fasts this long need a doctor. NICE guideline CG32
Zauner et al., Am J Clin Nutr, 2000 (11 adults)The app
The body is the screen.
No rings, no badges, no feed. A timer, a body and the science behind each hour, on one screen.
Start when you finish eating.
Pick 12, 16 or 18 hours, or set your own from 8 to 72. Drag the timeline to see what any hour ahead looks like.
Every fast, kept.
Your streak, your longest fast, the last seven days, and a weight log if you want one. Forgot to start the timer? Add the fast afterward.
It stays on your phone.
No account. No sign-up. Your fasts and your weight never leave your device.
Every figure has a source.
Each hour’s text links to the study behind it, and says how many people were in it.
Straight answers
What it won’t tell you.
Most fasting apps promise things the research doesn’t show. This one doesn’t.
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It won’t promise weight loss.
Eating in an 8-hour window lost no more weight than eating through the day over 12 weeks, and no more than ordinary calorie cutting over a year.
Lowe et al., JAMA Intern Med, 2020 (116 adults) Liu et al., N Engl J Med, 2022 (139 adults)
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It won’t tell you when “autophagy” starts.
In people, nobody has shown an hour when fasting switches on your cells’ clean-up.
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It doesn’t measure your body.
It shows what typically happens in a healthy adult. The timing varies from person to person.
Fasting isn’t safe for everyone.
Don’t fast without talking to your doctor first if you:
- are under 18
- are underweight
- have, or have had, an eating disorder
- are pregnant or breastfeeding
- take diabetes medicine, or any medicine that must be taken with food
- have a history of low blood sugar
Johns Hopkins Medicine Cleveland Clinic Harvard T.H. Chan School of Public Health
Coming soon to iPhone.
How You Tick Fasting is on its way to the App Store.