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HOME / FASTING / WATER FASTING
Water only, for a set number of hours or days. No food, no calories, and nothing else in the glass.
Burn times it and names each stage you reach. Anything past 24 hours needs planning, and three days or more needs a doctor.
What counts as water fasting
Intermittent fasting narrows when you eat. Water fasting removes food entirely for the duration, which is why the same hours feel different and the planning matters more.
IN THE GLASS
Water, still or sparkling
Salt
Electrolytes without calories
Plain water alone can make you feel worse on a long fast, because lower insulin means your kidneys release sodium and fluid.
NOT IN THE GLASS
Food of any kind, juice, milk, protein
Coffee and tea, on a strict water fast
Diet drinks and sweeteners
Black coffee and plain tea carry no meaningful calories, so they do not break a fast in general. On a water fast the definition is stricter, and the choice is yours to make.
The timeline
Hover any band to isolate it.
The first day runs at one scale and everything after 24 hours is compressed against it at three to one, so the early hours stay readable. Nothing in the biology switches on at a stated hour; Burn draws each line because a timer has to draw one.
By length
Each length has its own demands. The stage you reach is the smaller part of the difference; the planning is the larger one.
24h
TO THE FAT BURNING LINE
Dinner to dinner, so you never spend a whole calendar day without eating. The usual starting point.
72h
TO THE AUTOPHAGY BOUNDARY
Where medical supervision stops being optional, and where how you break it matters as much as the fast.
120h
DEEP AUTOPHAGY
Well past the point where this is a personal experiment. Planned with a doctor, including the refeed.
168h
DEEP AUTOPHAGY
The longest span Burn's stage model covers. Medical territory, and the refeed is the risk.
EVIDENCE
Which claims are well established, which have good human trials, and which are still active research.
SAFETY
The symptoms that mean stop, refeeding syndrome, and the conditions that rule this out entirely.
HOW TO
What to do in the days before, how to handle the hard hours, and how to come off it safely.
Before you start
Burn is a fasting timer. It is not medical advice, and it does not diagnose or treat anything.
DO NOT WATER FAST
You are pregnant or breastfeeding, you have a history of eating disorders, or you are under 18.
SPEAK TO A DOCTOR FIRST
Diabetes, medication timed to meals, blood pressure, kidney function or heart medication all change what a long fast does to you.
STOP AND EAT
Dizziness that does not settle, heart palpitations, shaking or confusion mean end the fast. None of those are stages to push through.
Fasts of three days or more should be planned with a doctor, including how you break them. Read the safety guidance →
Running one in Burn
A water fast rarely has a length you can commit to in advance. Burn's Flexi plan runs open-ended, naming each stage as you reach it with no goal to fall short of, which suits a fast you intend to end when your body says so rather than when a countdown does.
Water, sodium, potassium and magnesium all sit in the Journal, so a long fast has a record of what you actually took rather than a vague memory.
Six tools one tap from the timer, including a hydration prompt and a symptom check.
Blood Levels stores ketone and glucose numbers you have measured yourself. Burn holds what you record; it does not estimate it.
Burn maps elapsed time to named stages and records what you log. It does not measure ketosis, autophagy, or metabolic state. Onset of each shift varies with your last meal, your activity, your sleep and your metabolism.
Claims across this section are drawn from peer-reviewed research and traced in our evidence policy. Where human data is thin, the page says so rather than upgrading the claim.
Burn names every stage you reach, logs your water and electrolytes, and keeps the record of what you actually did.
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