HOME / FEATURES / EXTENDED FASTING
Four of Burn's nine stages sit beyond 24 hours. The timer switches to days, the stage names keep going to a full week, and the guidance changes with them.
Past the first day
What changes in the app
Past 24 hours the timer switches to days, hours and minutes, so 3d 6h 42m is legible where 78 hours 42 minutes is not.
Boundaries at 48, 72 and 96 hours appear on the stage rail, so a long fast still shows the next line ahead of it rather than running off the end.
Sodium, potassium and magnesium sit in the Journal next to your water, because a multi-day fast means no sodium arriving with meals.
If you measure ketones or glucose, Blood Levels stores what you record. Burn holds those numbers; it does not estimate them.
Running one without a target
A daily schedule has a fixed target. A long fast can be decided hour by hour, which is the situation open-ended mode is built for. The clock counts, the stages are named as you reach them, and you end it when you decide to rather than falling short of a number.
However it ends, the fast is recorded at the length it reached, with the stage it got to.
BEFORE A LONG FAST
Fasts of three days or more should be planned with a doctor, including how you break them. Do not fast at all if you are pregnant or breastfeeding, have a history of eating disorders, or are under 18, and speak to your doctor first if you manage diabetes or take medication timed to meals.
Stop and eat if you feel unwell, whatever the timer says. Dizziness that does not settle, heart palpitations or shaking are a different category from ordinary hunger.
The fuel sequence across a long fast is well described. Human timing and scale for autophagy and cellular repair are active research areas, much of that work coming from animal models. Claims are drawn from peer-reviewed research and traced in our evidence policy.
Burn maps elapsed time to named stages and does not measure ketosis, autophagy, or metabolic state. Onset varies with your last meal, your activity, your sleep and your metabolism, so each boundary is a line a timer has to draw.
Stage names to 168 hours, electrolytes logged alongside, and the whole thing recorded at the length it reached.
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