Last updated: March 2026
What Is Intermittent Fasting?
Intermittent fasting (IF) is a dietary approach that cycles between periods of fasting and eating. Unlike traditional diets that restrict what you eat, IF restricts when you eat. The most common method is 16:8 — fasting for 16 hours and eating within an 8-hour window.
This free timer tracks your fasting window with a live countdown, shows research-backed body milestones at each fasting stage, and logs your completed fasts. No app download, no account, no subscription. Select your protocol and start.
The timer supports 16:8, 18:6, 20:4, OMAD (23:1), and fully custom fasting/eating splits. Backdate your start time if you already stopped eating, and get browser notifications when your eating window opens.
How Intermittent Fasting Works
Hours 0-4: Your body processes your last meal. Blood sugar rises, insulin is released, and nutrients are absorbed. By hour 4, blood sugar returns to baseline.
Hours 4-12: Glycogen stores in the liver are gradually depleted as your body draws on stored energy. By hour 12, your body begins shifting to fat oxidation as the primary fuel source.
Hours 12-18: Ketosis begins. Fat is broken down into ketone bodies for energy. Autophagy — the cellular cleanup process that recycles damaged proteins — ramps up around 16-18 hours. Growth hormone levels rise significantly.
Hours 18-24+: Deep autophagy state. Cellular renewal and repair processes are at their peak. Extended fasts beyond 24 hours should be approached with medical guidance.
Choosing Your Fasting Protocol
16:8 (Leangains). The standard recommendation for beginners. Skip breakfast, eat from noon to 8pm. Since 7-8 hours of fasting happen during sleep, you only need to fast through the morning. Sustainable long-term for most people.
18:6. Slightly more restrictive. A 6-hour eating window (e.g., 1pm to 7pm) gives your body more time in the fat-burning zone while remaining manageable for most people with some IF experience.
20:4 (Warrior Diet). A 4-hour eating window requires careful meal planning to get adequate nutrition. Suitable for experienced fasters who are comfortable with longer fasting periods.
OMAD (23:1). One meal a day. The most restrictive daily protocol. Requires eating your full daily calories in a single sitting. Not recommended for beginners or those with a history of disordered eating.
Frequently Asked Questions
What is the best intermittent fasting schedule for beginners?
16:8 is the most popular starting point — fast for 16 hours and eat within an 8-hour window (e.g., skip breakfast, eat from noon to 8pm). It's manageable because 7-8 hours of fasting happen while you sleep.
How many hours of fasting before ketosis begins?
Ketosis typically begins around 12 hours into a fast, when your body depletes its glycogen stores and starts burning fat for fuel. By 16-18 hours, fat oxidation rates increase significantly. Individual timing varies based on activity level and diet.
What is autophagy and when does it start during fasting?
Autophagy is your body's cellular recycling process — damaged proteins and organelles are broken down and reused. It begins ramping up around 16-18 hours into a fast and increases significantly at 24+ hours. It's one of the primary health benefits of extended fasting.
Is it okay to drink water during intermittent fasting?
Yes — water, black coffee, and plain tea are fine during the fasting window. They contain zero calories and don't break your fast. Avoid anything with calories, artificial sweeteners (debated), or cream during fasting hours.
How do I track my intermittent fasting for free?
Use this timer — select your protocol, start the timer when you begin fasting, and the tool tracks your progress, shows body milestones, and logs your completed fasts. No app download or account needed.