From hour 8
Fat burning
With glycogen largely spent, stored fat becomes the main fuel.
- Typical onset
- 8–14 h
- Varies most with
- Sleep · activity · last meal
- Measurable at home
- No
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.
Sources
- Effects of intermittent fasting on health, aging and disease de Cabo R, Mattson MP · N Engl J Med · 2019 · Review
- Fasting: molecular mechanisms and clinical applications Longo VD, Mattson MP · Cell Metab · 2014 · Review
- Metabolic effects of intermittent fasting Patterson RE, Sears DD · Annu Rev Nutr · 2017 · Review