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HOME / FASTING / FASTING STAGES
A fast isn't one long wait. Your body switches fuel several times along the way, and each switch feels different. This page is every stage Burn tracks, what happens in it, and what you're likely to notice. Run an open-ended fast with no set finish and the timer simply keeps going.
How to read this
Nothing in your body flips on at an exact hour. It shifts with your last meal, your sleep, how much you moved. Burn marks the boundary at 14 hours as a guide, then tells you how firm the science behind each stage is. That is the part other timers skip.
9
stages, each with its own detail screen in the app
168h+
the ninth stage runs to seven days, and an open-ended fast keeps counting past it
5
stages you can reach on an ordinary daily schedule
2
stages that need medical clearance before you try them
Your everyday fast
You can reach all of these on a normal day, with a normal job and a normal dinner. If you've ever skipped breakfast, you've already been through the first three.
What's happening
Your body is digesting and absorbing nutrients. Blood glucose and insulin are elevated. No fat burning is occurring. Your glycogen stores are being topped up.
You may feel
Satisfied, energized, possibly sleepy after a large meal.
What's happening
Digestion is complete. Insulin levels are falling. Your body begins drawing on liver glycogen to maintain blood sugar. This is the foundation of every fast.
You may feel
Normal, possibly the first hints of hunger around the 4-5 hour mark.
What's happening
Liver glycogen stores are running low. The body begins gluconeogenesis, manufacturing glucose from lactate, glycerol and amino acids. Hunger signals may increase. This is temporary and will pass.
You may feel
Increased hunger, possible irritability, difficulty concentrating. This is the hardest window for most people.
The timer pill reads FAT RELEASE
What's happening
Insulin is now low enough that stored body fat begins to be released into the bloodstream as free fatty acids. The liver starts converting these into ketone bodies. You are transitioning into fat-burning mode.
You may feel
Hunger may intensify briefly, then begin to fade. Light-headedness is possible, so stay hydrated.
The timer pill reads KETOSIS
What's happening
Ketone production is meaningful. The brain begins supplementing its glucose use with ketones. Many people experience a mental clarity shift at this stage. Hunger often subsides.
You may feel
A shift in mental clarity. Hunger often drops noticeably. Some experience a mild euphoria.
A 16:8 gets you to Fat Mobilisation. An 18:6 gets you to Ketosis Entry. You don't need to go further than that to see results, and you shouldn't go past stage seven without a doctor.
Pick a schedule →If you go longer
Past 24 hours you are into extended fasting, where a set end time often stops making sense. Run the fast open-ended and Burn keeps counting, naming each stage as you reach it, with no target to fail. It tracks these stages because people do them, not because it recommends them. The last two need medical clearance.
What's happening
Your primary fuel source is now body fat. Ketone levels are elevated. Growth hormone has increased significantly, helping preserve lean muscle. Cellular stress response pathways are activating.
You may feel
Sustained energy, clearer thinking, reduced appetite. You may notice your breath changes slightly.
What's happening
The brain is running predominantly on ketones. Appetite suppression is significant. Insulin is at its lowest. Metabolic adaptation is in full effect. This is the sweet spot for extended fasters.
You may feel
Very little hunger. High mental clarity. Possible metallic taste. Strong sense of calm.
What's happening
Cellular autophagy is strongly upregulated. The body is breaking down and recycling damaged cellular components. This is the regenerative, anti-aging phase. Inflammation markers are reduced.
You may feel
Deeply calm, clear-headed, minimal hunger. Some experience emotional processing or vivid dreams.
What's happening
Stem cell production may be upregulated. The immune system undergoes partial reset. This is an advanced stage only appropriate for experienced fasters who are medically cleared and properly supplementing electrolytes.
You may feel
Variable. Some feel extraordinary, others experience fatigue. Electrolyte balance is critical at this stage.
This is the last named stage. An open-ended fast carries on counting past 168h and stays here.
No checking required
Cross into a new stage and Burn says so: a notification on your phone, a buzz on your wrist, the ring changing color. Tap it and you get the full detail screen for that stage, the same text you've been reading here.
Notifications are yours to shape. Stage changes only, goal reached only, or nothing at all.
See which schedule reaches which stage →
Hour by hour
Each of these covers what is happening at that point in a fast, what you are likely to feel, and which schedule gets you there.
Finish dinner, start the timer, and by breakfast you'll have crossed three of these stages without doing anything but sleeping. Set a goal or leave it open-ended and decide as you go.
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