20/4 intermittent fasting: a 4-hour eating window
20/4 limits eating to four hours, often one main meal and a snack late in the day. It is a demanding protocol best kept for experienced fasters.
Plan my 20/4 fast
Before you start
Fasting is not for everyone. Tick anything that applies to you: your answers stay on this screen.
Your day over 24 hours
- Eating
- Fasting
- Sleep
- Eating
- 15:30 – 19:30
- Fasting
- 20h
- 8h of fasting while you sleep
- 12h of fasting while awake
- Last meal 3h before bed
Fasting timer
Tap when you finish your last meal: the timer keeps counting even if you close the page.
Your progress
No fasts saved yet. Start the timer after your last meal: your streak will appear here.
Your progress is saved only on this device, in your browser. Nothing is sent anywhere.
What happens while you fast
Rough markers in hours since your last meal: they vary with what you ate, how active you are and your body.
Digestion
0 to 4h
Well established
What we know
Glucose from your meal enters the blood and insulin rises to move it into cells. The body stores part of the surplus as glycogen in the liver and muscles.
What we don't know
The exact timing depends on the meal: a fatty or high-fibre dish is digested more slowly than a sugary drink.
The liver takes over
4 to 12h
Well established
What we know
Insulin comes back down. The liver releases glucose from its glycogen stores to keep blood sugar steady, and fat use gradually increases.
What we don't know
Hunger comes in waves, often at your usual meal times, rather than tracking your actual reserves.
Switching to fat
12 to 18h
Established, varies a lot
What we know
Liver glycogen has dropped a lot. The body draws more on fat and starts producing ketone bodies: mild ketosis is possible for some people.
What we don't know
Studies place the switch anywhere from 12 to 36 hours: there is no guarantee it happens at 16 hours for you.
Mild ketosis
18 to 24h
Established, varies a lot
What we know
Ketone levels keep rising but stay far lower than after several days of fasting or on a strict ketogenic diet.
What we don't know
Often-quoted benefits (mental clarity, "detox") have not been shown in humans for fasts of this length.
Extended fasting
After 24h
Mostly in animals
What we know
Beyond 24 hours, the body makes more glucose from sources other than glycogen, and ketones rise clearly.
What we don't know
Autophagy (cell "recycling") is mostly measured in animals: we do not know when it increases in humans. Fasting for more than 24 hours needs medical supervision.
Sources: Cahill 2006; Rothman et al. 1991; Anton et al. 2018; de Cabo and Mattson 2019.
Your week
A plan that fits Thai habits. Tap a day to see the suggested meals.
140 hours of fasting this week
- 15:30First meal
- 19:00Last meal
- Family dinner fits in the window.
What can you drink while fasting?
Fasting cuts calories, not water. In the Thai climate, drinking regularly prevents many headaches and energy dips.
No problem
- Still or plain sparkling waterA glass when you wake up, then regularly.
- Black coffee, no sugarAmericano or kafae dam, without condensed milk.
- Unsweetened teaGreen, black or oolong tea, hot or iced.
- Unsweetened herbal drinksPandan leaf, lemongrass or ginger.
Keep it limited
- A splash of milk in coffeeA few calories: opinions differ, skip it for a strict fast.
- "Zero sugar" drinksNo calories, but they keep sweet cravings going.
This breaks your fast
- Milk tea, bubble tea, cha yenSugar and condensed milk.
- Sweet iced coffee, soft drinksSeveral spoonfuls of sugar per glass.
- Fruit juice, sweetened soy milkEven "natural" ones contain sugar.
Very hot or sweating a lot? A pinch of salt in your water or sugar-free electrolytes can help. Dizzy or unwell: eat and drink, the fast can wait.
Work out how much water to drink →Share my plan
This tool gives general guidance and does not replace medical advice. See a doctor or dietitian before fasting if you have a health condition or take medication.
Who is it for?
It only suits healthy adults already used to 16/8 or 18/6. It is not advised during pregnancy or breastfeeding, with treated diabetes, with an eating disorder or under 18, and should not be kept up for long without advice from a doctor or dietitian.
A sample day
If you wake at 06:30 and go to bed at 22:30, the eating window runs from 15:30 to 19:30: First meal at 15:30, Last meal at 19:00. The fast lasts 20 hours, 8 of them while you sleep.
Tips
It is hard to get enough protein, fibre and vitamins in four hours: make your meals very complete. Many people alternate 20/4 with 16/8 days rather than doing it every day.
These tools give general information. They do not replace the advice of a doctor or a qualified professional.
Frequently asked questions
- Is 20/4 dangerous?
- For a healthy adult, a few days are usually fine, but keeping it up for long makes it hard to meet your needs. Stop if you feel dizzy, unwell or unusually tired.
- What times work for 20/4?
- Usually late in the day, for example 15:30 to 19:30, so you keep dinner with the family.
- Does 20/4 lead to faster weight loss than 16/8?
- Not necessarily: at the same intake, studies show no clear advantage. A short window often means eating less, but it is also harder to stick to.