Fasting apps love throwing around terms like “autophagy” and “ketosis” to make you feel like you need a premium subscription to understand your own body.

You don’t.

Here’s what actually happens when you stop eating — hour by hour.

Hour 0-4: Fed State

You just ate. Your body is processing that food.

Blood sugar rises. Insulin spikes. Your body is in storage mode — taking nutrients from food and putting them where they belong.

What to know: This is normal. Nothing special happening yet.

Hour 4-12: Early Fasting

Blood sugar normalizes. Insulin drops.

Your body starts shifting from “use the food you just ate” to “use the energy you’ve stored.”

What to know: Most people hit this overnight without trying. It’s why you’re not starving when you wake up.

Hour 12-18: The Metabolic Switch

This is where it gets interesting.

Glycogen (stored sugar in your liver) runs low. Your body starts producing ketones — burning fat for fuel instead of glucose.

What to know: This is the “fat burning zone” everyone talks about. It’s real, not marketing.

Hour 18-24: Ketosis Deepens

Ketone levels rise. Your brain starts using them for fuel.

Many people report mental clarity here. Less brain fog. More focus.

What to know: This is why 18:6 and 20:4 protocols exist — to spend more time in this zone.

Hour 24-48: Autophagy Begins

Here’s the word everyone loves to throw around.

Autophagy = your cells eating their own damaged parts. Cellular cleanup. The body recycling old, broken cellular components.

What to know: This is real science, but the timing varies person to person. Don’t obsess over hitting exact hours.

Hour 48-72: Deep Fasting

Growth hormone increases. Stem cell regeneration begins. Your immune system starts resetting.

What to know: This is extended fasting territory. Not for beginners. Consult a doctor. Seriously.

The Practical Takeaway

You don’t need to fast for 72 hours to get benefits.

  • 16:8 gets you into early ketosis daily
  • 18:6 or 20:4 extends time in the fat-burning zone
  • 24+ hours is where autophagy really kicks in (occasional, not daily)

Most people get 80% of the benefits from simple 16:8 fasting done consistently.

Consistency beats intensity. Every time.

Track It Simply

You don’t need an app that shows you a “ketone level estimate” based on pseudoscience.

You need to know: How long have I been fasting? When can I eat?

That’s why I built FastTrack. One tap to start. Protocols built in. Streaks to keep you consistent. No PhD required.

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— Dolce