The science
Fasting stages, hour by hour
What your body may be doing from your last meal to 72 hours of fasting, and roughly when each stage begins
When you stop eating, your body doesn't switch fuels in an instant. It moves through a series of changes, and Fasting Tracker marks eight of them on its timer ring. Here is what each stage means and the hour it typically begins.
- 0 hBlood Sugar RisingYour body is digesting your last meal and using that energy. Insulin is elevated.
- 4 hBlood Sugar FallingBlood sugar settles back to normal and insulin starts to drop.
- 12 hFuel SwitchYour liver's glycogen is getting low, so your body starts turning to fat for fuel.
- 16 hFat BurningYour body starts burning stored fat for energy. Stay hydrated!
- 24 hKetosisFat is now your main fuel, and your liver turns more of it into ketones.
- 36 hAutophagyCells may start clearing out old components, a natural recycling process.
- 48 hDeep RenewalGrowth hormone levels may rise, helping to preserve muscle.
- 72 hImmune ResetExtended fasting territory. Only continue with medical guidance.
These hours are typical averages, not a schedule. Your last meal, how active you are and your metabolism all change the timing, and the later stages, from autophagy on, are still being studied in people.
Hour by hour
More guides
The metabolic switchHow your body moves from burning sugar to burning fat, when it happens and what speeds it up
Autophagy: your cells' recycling systemWhat autophagy is, what fasting does to it, and why nobody knows the exact hour it starts
Extended fasts (24–72 h): doing them safelyWhat happens in your body after the first day, who should avoid them, and how to start and finish well
A note on health
Fasting Tracker is designed for general wellness and is not a medical device. Fasting is not right for everyone. If you are pregnant, diabetic, have a history of eating disorders or take medication, talk to your doctor first, and before any extended fast.