Guide
What happens when you fast, hour by hour
What happens in your body when you fast, from your last meal to three days. These are the same stages Faaast shows on the timer, in plain words, and without the claims you’ll find in a lot of places online.
The stages at a glance
| Hours since you ate | Stage | What’s typically happening |
|---|---|---|
| 0–4 hours | Fed | You’re digesting your last meal. Blood sugar and insulin are up, and your body runs on the food you just ate. |
| 4–12 hours | Early fasting | Insulin falls and your body starts drawing on glycogen, the sugar stored in your liver and muscles. Most people are asleep for a lot of this stretch. |
| 12–18 hours | Fat burning | Your glycogen is running lower, so your body burns more fat for energy. This is where 16:8 ends. Hunger often comes in waves here and then settles. |
| 18–24 hours | Ketones rising | Your liver starts turning fat into ketones, a fuel your body and brain can use instead of sugar. |
| 24–48 hours | Ketosis | Ketone levels are clearly up and still rising. Your muscles and heart use more of them, and your brain starts to as well. This is where a 36-hour fast sits. |
| 48–72 hours | Extended | Deeper ketosis. Keep drinking water and take electrolytes, like a pinch of salt in water. If you feel dizzy, faint or unwell, end your fast. |
| 72 hours and more | Prolonged | Fasting this long should be done with a doctor’s guidance. When you end it, start with a small meal. |
These are typical timings, not a schedule. When each stage starts depends on what you last ate, how active you are, how long you’ve been fasting regularly, and your body. Research puts the switch from burning sugar to burning mostly fat somewhere between 12 and 36 hours,1 and light ketosis (about 0.5 mmol/L of ketones in the blood) usually arrives around 18 to 24 hours, sooner if you’ve exercised.2 That’s why Faaast only uses the word ketosis from 24 hours.
Why the first 12 hours feel easy
For most of this stretch your body is still living off your last meal and its stored sugar. If you stop eating after dinner, you sleep through a lot of it. This is why 12:12 is a good place to start.
12 to 18 hours: where most people stop
16:8 and 18:6 both end in this stage. Your stored sugar is running lower, so your body leans more on fat.1 It’s also where hunger tends to peak. It comes in waves, often around the times you’d normally eat, and then eases.3 Water, black coffee or tea help.
18 to 48 hours: ketones
Past about 18 hours your liver makes more ketones from fat. By the end of the first day you’re in light ketosis, and levels keep climbing for several days.2 The brain only gets a sizeable share of its energy from ketones after about three days: around a quarter by day three and a half.4 Longer fasts like 20:4, one meal a day (about 23 hours) and 36 hours reach into this stage. Drink plenty, and if you fast this long often, add electrolytes.
What about autophagy?
You’ll often read that autophagy, the body recycling old parts of its cells, “switches on” at 16, 24 or 48 hours. Most of that evidence comes from animal and cell studies; in humans it hasn’t been shown to start at a particular hour,5 so we don’t put it on a timeline and Faaast doesn’t claim it.
Past 48 hours, take more care
Longer fasts aren’t for everyone. Keep up water and electrolytes, and end the fast if you feel dizzy, faint, shaky or unwell. For very long fasts, health services advise medical supervision, and food should come back slowly: small meals rather than a big one.6 Faaast suggests a doctor’s guidance from 72 hours.
Seeing your stage in Faaast
With Faaast Pro, the timer shows the stage you’re likely in as you fast, from Fed to Ketosis and beyond, and can let you know when you reach the next one. You can log water and electrolytes with a tap right under the timer.
This guide is general information, not medical advice. Don’t fast without talking to a doctor if you’re pregnant or breastfeeding, under 18, have had an eating disorder, have diabetes, or take insulin or other medicine that lowers blood sugar.78
Questions
When does fat burning start when fasting?
Your body burns more fat once its stored sugar runs low. Research puts the switch between 12 and 36 hours after your last meal: it depends on what you ate and how active you are.
How long does it take to get into ketosis?
Most people reach light ketosis (about 0.5 mmol/L) somewhere around 18 to 24 hours, sooner after exercise. Faaast names it from 24 hours.
Do ketones fuel the brain after a day of fasting?
Only partly. Ketone levels rise through the first days, but even after three and a half days they cover about a quarter of the brain’s energy.
Does autophagy start at 16 hours?
That claim is common online, but most evidence comes from animal and cell studies, and in humans autophagy hasn’t been shown to start at a particular hour. Faaast doesn’t claim it.
Is it safe to fast for 72 hours?
Very long fasts are best done with medical supervision. Drink water, take electrolytes, end the fast if you feel dizzy, faint or unwell, and start eating again with small meals.
Sources
- Anton et al., Flipping the metabolic switch: understanding and applying the health benefits of fasting, Obesity, 2018. The switch from burning sugar to burning mostly fat “typically occurs between 12 to 36 hours” after your last meal.
- Deru et al., The effects of exercise on β-hydroxybutyrate concentrations over a 36-h fast, Med Sci Sports Exerc, 2021. Ketones reached 0.5 mmol/L after about 21 hours on average, or about 17.5 hours with exercise at the start.
- Natalucci et al., Spontaneous 24-h ghrelin secretion pattern in fasting subjects, Eur J Endocrinol, 2005. While fasting, the hunger hormone ghrelin still rises and falls around people’s usual mealtimes.
- Hasselbalch et al., Brain metabolism during short-term starvation in humans, J Cereb Blood Flow Metab, 1994. After 3.5 days of fasting, ketones covered about a quarter of the brain’s energy needs.
- Bagherniya et al., The effect of fasting or calorie restriction on autophagy induction: a review of the literature, Ageing Res Rev, 2018. Fasting induces autophagy in animal and cell studies; human evidence is limited.
- Refeeding syndrome, Cleveland Clinic. Prolonged fasting is best done under medical supervision, and food should be reintroduced slowly.
- Intermittent fasting, NHS (My Health London). Who shouldn’t fast: people with a history of eating disorders, and pregnant or breastfeeding women; people on medicine should talk to their GP practice first.
- Intermittent fasting: is the 5:2 diet good for weight loss?, British Heart Foundation. Who should avoid fasting: diabetes or low blood sugar, underweight, eating disorders, pregnancy or breastfeeding, children and teenagers.