Methodology

This page explains how the FastWindowTimer timer works, where the metabolic phase thresholds come from, and what their limitations are. The phases are general educational estimates drawn from published research — they are not a measurement of your individual body.

How the timer works

The timer records when you start your fast (stored in your browser's local storage) and counts elapsed time. When you set a protocol — for example, 16:8 — the timer shows how much of your fasting window has passed and how much remains. No data is sent to a server.

Metabolic phase thresholds

The hours below are general estimates drawn from published research on typical physiological responses to fasting. They are not precise for any individual — actual timing varies by prior meals, activity level, metabolic rate, body composition, and individual biology. The timer uses these as educational reference points only, not as personal metabolic measurements.

Phase	Typically from	What research suggests
Fed	Hour 0	Digestion and glucose metabolism; insulin elevated.
Early fasting	~Hour 4	Glycogen stores begin to deplete; body starts drawing on stored energy.
Fat burning	~Hour 12	Lipolysis typically increases as glycogen falls; fat oxidation rises for many people.
Ketosis	~Hour 16	Ketone production may begin in some individuals; onset varies widely.
Deep ketosis	~Hour 24	Elevated ketone levels possible for extended fasts; not typical for 16:8.

Limitations and assumptions

These thresholds are derived from population-level research, not individual measurement. They will not match everyone's experience. People with insulin resistance, athletes, or individuals on low-carbohydrate diets may enter fat-burning or ketosis earlier or later than the estimates suggest. Do not use these phases to make medical decisions. If you are managing a health condition, consult a clinician.

Published 2026-06-21. Sources reviewed by the FastWindowTimer editorial team. No named medical reviewer at this time.