What happens when you fast

What’s understood

Nutrients from the last meal are still being absorbed. Blood sugar rises, then settles as insulin does its ordinary work. Nothing about fasting has started yet.

What people commonly report

Usually nothing. Appetite at the accustomed mealtime is habit as much as need.

What it isn’t

Not a fasting state — this is how every gap between meals begins, whether or not a fast is intended.

Where the evidence is thin

Timing depends heavily on the size and makeup of the last meal; the 0–4 h range is an average, not a boundary anyone can feel.

What’s understood

With no incoming food, the liver releases stored glucose (glycogen) to hold blood sugar in its normal range. Insulin stays low. This is routine overnight metabolism.

What people commonly report

Hunger arriving in waves that pass rather than build. Some people feel a dip in energy around their usual mealtime.

What it isn’t

Not depletion — glycogen is drawn down gradually and refilled by the next meal. Nothing here needs pushing through.

Where the evidence is thin

How long stores last varies with activity and how much carbohydrate recent meals carried; the 4–8 h range is wide for a reason.

What’s understood

Liver glycogen is largely spent, so the body draws more of its fuel from stored fat. Insulin stays low. This is ordinary daily metabolism, not a special state.

What people commonly report

Hunger arriving in waves rather than building. Some notice steadier energy or clearer focus; others feel flat, cold or irritable. Both are usual and neither means it's working better.

What it isn’t

Not "fat loss in progress" — that depends on the whole day, not this hour. Not a switch that flips at hour 8. Your own timing shifts with sleep, activity and your last meal.

Where the evidence is thin

The switch from glucose to fat is gradual rather than a moment; no hour marks where it happens for one person.

What’s understood

Ketone production rises as fat becomes the dominant fuel. The switch from glucose is gradual rather than a moment, and its timing differs widely between people.

What people commonly report

Reduced appetite, a metallic or fruity taste, mild headache in the first few fasts, and for some a lift in concentration. Sleep is often lighter.

What it isn’t

Not a state this app can detect — ketosis is measurable only by testing blood or breath, never by a clock.

Where the evidence is thin

Ketone levels don't map neatly onto benefits, and nothing about reaching ketosis in a single fast has been shown to change long-term health on its own. We can't tell you whether you're in it — only what hour you're at.

What’s understood

In cell and animal studies, prolonged fasting increases the signalling for autophagy — the recycling of worn cell components. Human data under everyday fasting conditions is limited.

What people commonly report

Little that is specific to this phase. Ongoing hunger, lighter sleep and feeling cold are commonly described on longer fasts.

What it isn’t

Not something a phone can verify. We won't say your fast triggered autophagy — nothing in an app can know that, which is why the phase is named for the signalling, not the result.

Where the evidence is thin

The evidence is largely mechanistic — from cells and animals, not from people fasting overnight. Hour 18 marks where studies commonly look, not where anything begins.