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HOME / FASTING STAGES / 18 HOURS
Eighteen hours is where Burn moves you from Fat Mobilisation into Ketosis Entry. Ketone production becomes meaningful, and your brain starts supplementing the glucose it runs on with ketones instead.
It is also the hour an 18:6 schedule is built around.
Burn names the stage and tells you how firm the evidence behind it is.
Ketosis Entry appears as KETOSIS on the timer screen, where space is tight. Same stage, shorter name.
The stage itself
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.
Hunger dropping is the part people do not expect. The hard window was four hours ago, not now.
Your brain cannot run on fat directly, but it can run on ketones. That switch is what this stage describes.
A mental clarity shift is common at this stage, though how strongly you notice it varies a lot.
Longer fasts mean less sodium coming in with food. A pinch of salt in your water is the usual advice.
Getting here
An 18:6 finishes right at the boundary. A 20:4 or a longer fast passes through it on the way to Fat Burning.
18:6
Runs to the Ketosis Entry boundary. Go a little past your goal and you are into the stage itself.
The 18:6 guide →
20:4
Spends two hours inside Ketosis Entry before the window opens. One four-hour eating window, harder socially than physically.
All schedules →
What the clock can say
Three claims about this stage, at three different levels of certainty.
WELL ESTABLISHED
That the liver produces ketone bodies from fatty acids, and that levels climb the longer you go without food, is well described in the literature.
MEASURABLE WITH A TEST
A blood or breath test gives you a number. Burn names the stage the clock has reached, not a ketone reading.
VARIES BY PERSON
Onset shifts with your last meal, your activity, your sleep, and your metabolism, so 18 hours is a boundary in the app's model rather than a moment in your body.
Burn maps elapsed time to named stages and does not measure ketosis, autophagy, or metabolic state. Stop and eat if you feel unwell, whatever the timer says.
Getting through to it
Hunger arrives in waves that peak and pass. On a fast that reaches 18 hours, the wave that catches people lands hours earlier.
A lot of what people read as a fasting headache is fluid and sodium. Burn tracks your water alongside the fast.
Two normal meals in six hours, not a feast. Eating your way back through the deficit is what stops a longer fast achieving anything.
Burn names all nine stages as you reach them, with a buzz on your wrist at every boundary.
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