How long should I fast? A beginner's guide to picking a windowLonger is not automatically better. Here is a sane progression from a 12-hour overnight fast to longer windows, and the signals that tell you to stop.The most common beginner mistake in intermittent fasting is starting at 20 hours because it sounds serious, feeling terrible on day three, and concluding fasting does not work. A slower start almost always produces a longer practice.Start where you already areMost people already fast for 10–12 hours overnight without calling it that. If you finish dinner at 8pm and eat breakfast at 7am, that is an 11-hour fast. Your first goal is simply to make it deliberate and consistent.A four-week progression

Week 1 — 12:12. Fixed dinner cut-off, fixed breakfast. Nothing else changes.

Week 2 — 14:10. Push breakfast back by two hours. Water, black coffee and plain tea stay fine.

Week 3 — 16:8. The standard window. Hold it on weekdays and relax on one weekend day if you need to.

Week 4 — hold. Do not extend. Repeat week 3 and see whether it still feels sustainable.

Only consider 18:6 or longer after a full month at 16:8 feels unremarkable.What happens across a fastRoughly, and with wide individual variation:

0–4 hours — digestion and absorption.

4–8 hours — the body leans on stored glycogen.

8–12 hours — fat oxidation increases as glycogen falls.

12–16 hours — fat burning becomes the dominant fuel source for most people.

16 hours and beyond — cellular clean-up processes ramp up, though the human evidence here is far less precise than the internet suggests.

Signals to stop or shortenStop the fast and eat if you experience dizziness, faintness, shakiness, heart palpitations, confusion, or persistent poor sleep across several nights. Chronic irritability and a rising resting heart rate are quieter warnings worth taking seriously.Track two things, not tenFor the first month, log only your fast and your sleep. That is enough to see whether the window is helping or costing you. Quies records both on one daily view, so the trade-off is visible without extra effort.SafetyFasting is not suitable during pregnancy or breastfeeding, for under-18s, or for people with a history of disordered eating. If you take medication with food or manage a chronic condition, speak with your doctor first. Quies is a record-keeping and reflection app, not clinical guidance.More from the Quies Journal · Quies home