Under the dial in Step Step, you'll see five stages: Digesting, Blood sugar falling, Fat burning, Ketosis and Deep ketosis. They start at 0, 4, 12, 16 and 24 hours. So what do they mean?
Think of them as a rough map of what often goes on while you're not eating. At first, your last meal is still being digested. Later, blood sugar usually settles down. Then your body often leans more on stored fat, and for many people it starts making ketones, which are fuel it can make from fat.
The hour marks are landmarks, not switches. Everyone is different, and the timing can shift with what you ate, how active you've been and even how well you slept. Two people can start a fast together and be in quite different places at hour 14.
That's why the stages aren't targets. You don't have to reach Ketosis to have had a good fast, and hitting a stage isn't a prize. A gentle 13 hours that you enjoy beats a gruelling 20 that leaves you grumpy.
You'll also notice the guide stops at 24 hours. That's on purpose. Step Step doesn't want to nudge anyone toward longer fasts. Use the stages out of curiosity, listen to how you feel, and eat when it's time.
What are the fasting stages hour by hour?
The five stages in Step Step are Digesting from hour 0, Blood sugar falling from hour 4, Fat burning from hour 12, Ketosis from hour 16, and Deep ketosis from hour 24. Here is what each is loosely describing.
Hours 0 to 4: your body is still working through your last meal. Blood sugar and insulin are higher, and the food you ate is being used or stored.
Hours 4 to 12: digestion finishes, and blood sugar usually settles. Your body starts drawing on stored energy, including glycogen in the liver. You may feel hunger come and go.
Hours 12 to 16: stored carbohydrate is running lower and the body leans more on fat. Hours 16 and beyond: some people begin to make ketones in noticeable amounts. By 24 hours, this is more established, though again it varies a lot.
Treat this as a rough sketch. Real bodies blur the edges, and nothing dramatic happens at the stroke of an hour.
Hours 0 to 4 are the stage most people forget about. You are not fasting yet in any meaningful sense, which is why the clock on a fast often counts from your last bite rather than from when you feel hungry. If you are tracking, that simply means starting the timer when you finish eating.
How accurate are the fasting stage timings?
They are approximate. The numbers are an average idea drawn from physiology textbooks and small studies, not a stopwatch for your own body.
A big dinner with plenty of carbohydrate, a hard workout, a poor night of sleep, your age, and how often you have fasted before can all move the timings by hours. Someone who is used to fasting may start making ketones earlier than someone doing their first fast.
There is also no clear test you can feel. Many people do not notice any difference at hour 12 or hour 16. Do not worry if the stage on screen does not match how you feel.
One more thing worth knowing: people who eat very little carbohydrate in general may reach the later stages sooner, while a high-carbohydrate dinner can push things back. This is another reason to read the labels as a loose guide.
Do you need to reach ketosis to benefit from fasting?
No. Most of the research on time-restricted eating looks at windows of 8 to 14 hours, which sit well before ketosis for many people. Some people find a regular overnight break of 12 to 14 hours helpful just for routine, with no late-night snacking.
It is easy to turn the stages into a ladder to climb. Try not to. Stopping at hour 13 because you feel good is a perfectly reasonable choice, and stopping at hour 10 because you are hungry is too. A fast that stays enjoyable is much more likely to become a habit you can keep.
If you want something to look at while you wait, notice how you feel rather than which stage is on screen: energy, mood, hunger and sleep tell you more than any label.
Why does the guide stop at 24 hours?
Because longer fasts carry more risk and need medical guidance. They can cause dizziness, low blood pressure, headaches and electrolyte problems, and they are not suitable for many people. Step Step is built for gentle fasting, so it does not encourage stretching further.
If you are curious about the stages for their own sake, that is fine. Use them as a story about what your body might be doing, then eat when you are ready.
Common questions
What happens to your body after 12 hours of fasting?
By around 12 hours, digestion is done and blood sugar has usually settled. Your body is drawing more on stored energy. Most people feel mostly normal, perhaps a little hungry.
When does ketosis start during a fast?
For many people it begins somewhere after 16 hours, but it varies with your last meal, activity and body. It is not a switch, and you do not need to reach it for a fast to be worthwhile.
Is it safe to fast for 24 hours?
Many healthy adults manage a 24 hour fast, but it is not right for everyone. Anyone who is pregnant, under 18, has diabetes, takes medication or has a history of disordered eating should speak to a doctor first.
Do the fasting stages mean I am burning fat?
Your body uses some fat for energy at all times. The stages describe a gradual shift in fuel mix, not an on and off switch, and they do not predict weight change.
Updated October 25, 2026
This is general information, not medical advice. Check with your doctor before you start fasting, especially if you have a health condition, are pregnant or take medication.
