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5:2 Protocol: Eat normally for 5 days per week and restrict calories to 500–600 on 2 non-consecutive days. This protocol doesn't use a timed fast — track your restrict days manually.
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Fasting Stages
Fed State
0 – 4 hours
Body digests and absorbs food. Insulin levels are elevated, storing energy.
Early Fasting
4 – 8 hours
Blood sugar drops, insulin decreases. Body starts using stored glycogen for fuel.
Fat Burning
8 – 12 hours
Glycogen depleted. Fat oxidation increases as the body shifts to burning stored fat.
Ketosis
12 – 18 hours
Liver produces ketone bodies. Brain begins using ketones for fuel alongside glucose.
Deep Ketosis
18 – 24 hours
Peak fat burning and autophagy — cellular cleanup and repair processes accelerate.
Fast History
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How the Intermittent Fasting Timer Works
This free intermittent fasting timer helps you track your fasting window with a live countdown, visual progress ring, and real-time fasting stage indicators. Choose your protocol, set your start time, and let the timer guide you through your fast.
What is intermittent fasting?
Intermittent fasting (IF) is an eating pattern that cycles between periods of fasting and eating. Unlike traditional diets, IF doesn't restrict what you eat — it restricts when you eat. The most popular method is 16:8 fasting, where you fast for 16 hours and eat within an 8-hour window each day.
Popular intermittent fasting protocols
- 16:8 (Leangains): Fast 16 hours, eat within 8 hours. The most popular and sustainable protocol for beginners. Most of the fast happens while you sleep.
- 18:6: Fast 18 hours, eat within 6 hours. A step up from 16:8 that increases time in fat-burning and early ketosis states.
- 20:4 (Warrior Diet): Fast 20 hours, eat within a 4-hour window. Reaches deeper ketosis. Best for experienced fasters.
- OMAD (23:1): One Meal a Day — fast for 23 hours and eat everything in a single meal. Maximum autophagy and ketosis benefits.
- 5:2: Eat normally 5 days per week, restrict to 500–600 calories on 2 non-consecutive days. Flexible and doesn't require daily time-restricted eating.
The five stages of fasting
As your fast progresses, your body moves through distinct metabolic stages. During the fed state (0–4 hours), your body digests food and insulin is elevated. In early fasting (4–8 hours), blood sugar drops and your body taps glycogen stores. Fat burning kicks in at 8–12 hours as glycogen depletes. Around 12–18 hours, you enter ketosis where the liver produces ketone bodies. Beyond 18 hours, deep ketosis and autophagy — your body's cellular cleanup process — are in full effect.
Tips for successful intermittent fasting
- Start with 16:8 and gradually increase your fasting window over weeks
- Stay hydrated — drink water, black coffee, and plain tea during fasts
- Break your fast with easily digestible foods, not a heavy meal
- Keep busy during fasting hours — hunger often comes in waves and passes
- Be consistent with your eating window to help your body adapt
- Track your fasts to build accountability and see your progress
Frequently Asked Questions
What is 16:8 intermittent fasting?
16:8 intermittent fasting means you fast for 16 hours and eat within an 8-hour window each day. For example, if you eat your last meal at 8 PM, you wouldn't eat again until 12 PM the next day. It's the most popular IF protocol because it's sustainable — most of the fasting happens while you sleep.
What can I drink while fasting?
During your fasting window you can drink water, black coffee (no sugar, no cream), plain tea, and sparkling water. These have zero or negligible calories and won't break your fast. Avoid diet sodas with artificial sweeteners as some may trigger an insulin response, though research is mixed.
How long does it take to enter ketosis while fasting?
Most people begin producing ketones after 12–18 hours of fasting. By 18–24 hours, you're typically in a deeper state of ketosis where your body is primarily burning fat for fuel. The exact timing depends on your glycogen stores, activity level, and what you ate before starting the fast.
Will intermittent fasting slow my metabolism?
Short-term fasting (up to 24–48 hours) does not slow your metabolism. In fact, studies show fasting periods under 72 hours can slightly increase metabolic rate due to norepinephrine release. Metabolic slowdown is associated with prolonged caloric restriction over weeks, not daily time-restricted eating.
Is it okay to exercise while fasting?
Yes, moderate exercise during a fast is generally safe and may enhance fat burning. Light cardio, yoga, and walking are well-tolerated. For intense strength training, some people prefer to train near the end of their fast or during the eating window to have fuel for recovery. Stay hydrated and listen to your body.
What is OMAD (One Meal a Day)?
OMAD stands for One Meal a Day — a 23:1 fasting protocol where you eat all your daily calories in a single meal within a 1-hour window. It's an advanced form of intermittent fasting that maximizes time in ketosis and autophagy. OMAD is not recommended for beginners; start with 16:8 and work up gradually.
What are the stages of fasting?
Fasting progresses through stages: Fed state (0–4 hours) where your body digests food; Early fasting (4–8 hours) when blood sugar drops and insulin decreases; Fat burning (8–12 hours) as glycogen depletes and fat oxidation increases; Ketosis (12–18 hours) when the liver produces ketone bodies; and Deep ketosis (18–24+ hours) with peak fat burning and autophagy (cellular cleanup).
How do I break a fast properly?
Break your fast with easily digestible foods. Start with something small like bone broth, a handful of nuts, or some avocado. Avoid immediately eating a large, heavy meal — this can cause bloating and digestive discomfort. Wait 15–30 minutes after your first small snack before eating a full meal.