A fast is not one long wait. Your body moves through recognisable stages, and knowing which one you are in makes the hours easier.
Short answer
The stages of fasting are roughly: fed and digesting (0 to 4 hours), blood sugar settling (4 to 12 hours), fat burning begins (12 to 16 hours), ketosis ramping up (16 to 24 hours), and deeper ketosis after 24 hours. Exact timing varies from person to person.
The fasting timeline, hour by hour
0 to 4 hours: fed and digesting. Your body is breaking down your last meal and using it for energy.
4 to 12 hours: blood sugar settling. Digestion finishes, blood sugar and insulin come down, and your body starts drawing on stored glycogen.
12 to 16 hours: fat burning begins. As glycogen runs lower, your body turns more to stored fat for fuel.
16 to 24 hours: ketosis ramping up. The liver makes more ketones from fat, which the body and brain can use as fuel.
24 hours and beyond: deeper ketosis. Ketone levels keep rising. Fasts this long are advanced and not needed for a daily routine.
These boundaries are approximations. Your last meal, activity, sleep and your own metabolism all move them. Treat the timeline as a guide, not a measurement.
What happens after 16 hours of fasting?
By 16 hours most people have moved from running mainly on their last meal to running more on stored fat, and ketone production is picking up. This is why 16:8 is such a common plan: it reaches this point daily without asking for more.
How the stages feel
Hunger usually peaks around your normal meal times, not steadily upwards.
Many people feel a dip around hours 12 to 14, then feel clearer afterwards.
Bide puts the timeline on the timer itself, so you always know where you are.
1
Start a fast
The ring begins to fill, with a marker for each stage along it.
2
Read the current stage
Under the timer you see the stage you are in right now, such as "Fat burning begins".
3
Open Body Status
Tap the stage for a full-screen explanation of every stage, written in plain language.
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Common questions
For most people the shift towards burning more stored fat begins somewhere around 12 hours after the last meal. It is gradual, not a switch.
Ketone production usually starts to rise between 12 and 18 hours into a fast and keeps building. It depends on what you ate before and how active you are.
Yes. Eating a meal returns you to the fed stage, and the timeline starts again with your next fast.
Sources
The studies and health guidance this guide draws on.
This guide is general information, not medical advice. Talk to a doctor before fasting if you are pregnant or breastfeeding, under 18, have a health condition or take medication.
