When does fat burning start during fasting?

Fat burning begins around 8–12 hours into a fast when liver glycogen stores are depleted. Ketosis (using fat as primary fuel) starts at approximately 12–14 hours.

When does autophagy start during fasting?

Autophagy begins mildly at around 12 hours and reaches significant levels at 16–18 hours of fasting. Deep autophagy occurs from 24 hours onward.

What happens to your body in the first 4 hours of fasting?

In the first 4 hours, your body digests your last meal, absorbs nutrients, and insulin peaks then falls. Blood sugar returns to baseline and cells are fuelled by glucose.

How long until you are in ketosis when fasting?

Ketosis typically begins at 12–14 hours of fasting, when liver glycogen is nearly depleted and the liver begins converting fatty acids to ketone bodies.

What happens if you fast for 36 hours?

At 36+ hours, you experience deep autophagy, significant growth hormone release, immune system modulation, and maximum ketone production. This is extended fasting territory — consult a doctor before attempting.

The 8 Stages of Fasting — What Happens Inside Your Body

Understanding what happens inside your body during a fast is one of the most powerful motivators for sticking to your protocol. Here is a science-backed, hour-by-hour timeline.

0–2 Hours

Digestion Phase

Your body processes the nutrients from your last meal. Glucose enters the bloodstream, insulin is released, and nutrients are directed to cells and glycogen storage in the liver and muscles.

2–4 Hours

Blood Sugar Stabilising

Insulin levels peak and begin to fall. Blood sugar returns toward baseline as cells absorb glucose. Energy feels stable. Your body is still running on food fuel.

4–8 Hours

Glycogen Burning

With no new food coming in, your body draws on liver glycogen stores for energy. This phase is the classic "hunger window" for many people — ghrelin (the hunger hormone) peaks here.

8–12 Hours

Glycogen Depletion

Liver glycogen stores are nearly exhausted. Free fatty acids are mobilised from fat tissue. Your body prepares to shift fuels from glucose to fat.

12–14 Hours

🔥 Ketosis Begins

The liver begins converting fatty acids into ketone bodies (beta-hydroxybutyrate, acetoacetate). Your brain switches to ketones as its primary fuel. Fat burning begins in earnest. Source: Cahill GF Jr, NEJM 1970.

14–16 Hours

✨ Early Autophagy

Cellular housekeeping begins. Autophagy (self-eating) is activated — damaged proteins and organelles are tagged for recycling by lysosomes. This is the "repair mode" of fasting that 16:8 practitioners aim for.

16–24 Hours

Peak Autophagy

Autophagy reaches significant levels throughout the body. Growth hormone pulses increase, protecting muscle mass. The cellular repair process accelerates. Source: Levine & Kroemer, Cell 2008.

24–36+ Hours

Deep Autophagy & Immune Reset

Maximum autophagy. Significant immune system modulation begins — stem cells are activated. Ketones reach high levels. Inflammation markers decrease. This is extended fasting territory. Source: Longo & Mattson, Cell Metabolism 2014.

Which Fasting Protocol Hits Which Stages?

Protocol	Ketosis	Early Autophagy	Peak Autophagy
12:12	❌	❌	❌
16:8	✅ Yes (at end)	✅ Yes	❌
18:6	✅ Yes	✅ Yes	Partial
20:4	✅ Yes	✅ Yes	✅ Yes
OMAD	✅ Yes	✅ Yes	✅ Yes