A fast is measured from the last calorie to the next one. Finish dinner at 20:00 and a 16-hour fast ends at 12:00 the next day, giving an eating window of 12:00 to 20:00.
| Protocol | Fast | Eating window | Last meal 20:00 → first meal |
|---|---|---|---|
| 12:12 | 12 h | 12 h | 08:00 |
| 14:10 | 14 h | 10 h | 10:00 |
| 16:8 | 16 h | 8 h | 12:00 |
| 18:6 | 18 h | 6 h | 14:00 |
| 20:4 | 20 h | 4 h | 16:00 |
| OMAD | ~23 h | ~1 h | 19:00 |
Separate from these are the day-based patterns: 5:2, where two non-consecutive days are restricted to around 500 to 600 kcal rather than zero, and alternate-day fasting, which alternates a normal day with a very low intake day.
Roughly, and with wide individual variation:
You will see confident claims that autophagy — the cellular recycling process — switches on at a specific hour, often quoted as 16 or 18. That number is not established in humans. The underlying research is largely from yeast, cell cultures and rodents, and rodents metabolise several times faster than we do, so their timelines do not transfer. Autophagy is a real and continuous process that fasting appears to upregulate; the precise human threshold has not been measured non-invasively, and any tool that displays an autophagy countdown is displaying a guess.
The honest summary is less dramatic than the marketing. Randomised trials and meta-analyses comparing intermittent fasting with conventional continuous calorie restriction have generally found similar weight loss when total calories are matched. Some trials found modest additional benefits for insulin sensitivity and blood pressure; others found none beyond what the weight loss explained.
That is not a reason to dismiss it. It means intermittent fasting is best understood as a calorie-control strategy rather than a metabolic trick — and for many people it is a genuinely easier one, because a rule about time is simpler to follow than a rule about every portion. If skipping breakfast means you eat 400 fewer calories without tracking anything, that is a real result. If it means you eat 900 calories at 22:00, it is not. One trial-level finding worth knowing is that very short eating windows sometimes led to reduced lean mass, which argues for adequate protein and resistance training rather than for longer fasts.
This part is not optional. Fasting is inadvisable or requires medical supervision if you are pregnant or breastfeeding, under 18, have a history of an eating disorder, are underweight, have type 1 diabetes, or take insulin or sulfonylureas — where fasting carries a real hypoglycaemia risk and doses often need adjusting. It also interacts with medications that must be taken with food. Speak to a doctor or pharmacist before starting, and stop if you experience dizziness, palpitations or persistent weakness. Nothing on this page is medical advice.
A timer measures elapsed time. It cannot know what you ate before it started, whether your total intake is appropriate, whether the coffee you had counts, or how your individual physiology responds. It also cannot see the failure mode that ends most attempts, which is not hunger during the fast but overeating after it.
Black coffee, plain tea and water contribute essentially no calories and are generally considered fine. Milk, sugar and sweetened creamers do add calories and an insulin response. Non-caloric sweeteners are debated and the evidence is mixed.
Not necessarily. The window that you can keep for months beats the stricter one you abandon in a fortnight. Many people find 14:10 sustainable indefinitely and 20:4 impossible.
Low-intensity work fasted is fine for most people. Heavy or long sessions usually go better fed, and performance in high-intensity efforts is often measurably worse without carbohydrate available.
No. Your start and end times are kept in your browser only and nothing is transmitted or saved.
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