How many hours of not eating is considered a fast?

November 24, 2025 •

4 min read

According to some researchers, fasting for at least 12 hours is required to induce metabolic changes, although longer fasts increase these effects. A fast is defined simply as a period of abstaining from food, and various timeframes offer distinct benefits depending on the duration and individual goals.

Quick Summary

Intermittent fasting involves abstaining from calories for a set period, typically for at least 12 hours, to trigger metabolic changes like fat burning. Popular methods include 16:8 and 5:2, each offering different health benefits depending on duration.

Key Points

12 Hours is the Minimum: Scientifically, a fast begins after about 12 hours of not eating, as this is when the body starts shifting its energy source.
16 Hours for Increased Fat Burning: Extending the fast to 16 hours, known as the 16:8 method, promotes greater depletion of glycogen and more significant fat-burning.
Longer Fasts Activate Autophagy: Fasting for 24 hours or more triggers the cellular repair process called autophagy, where the body recycles damaged cells.
Stay Hydrated During Fasting: Drinking plenty of water, black coffee, or tea is essential during fasting periods to stay hydrated and manage hunger.
Consistency is Key: The most effective fasting method is one that fits your lifestyle and that you can maintain consistently for long-term health benefits.
Consult a Professional: Individuals with health conditions like diabetes, pregnant or breastfeeding women, and those with a history of eating disorders should consult a doctor before fasting.

What is a Fast and How Does Your Body React?

In its simplest form, a fast is any period without food or caloric intake. While most people fast naturally overnight, intermittent fasting extends this period to trigger various metabolic responses. During a fast, your body goes through several phases as it adapts to using different energy sources.

The Initial Phase (0-12 hours): In the first few hours after eating, your body uses the glucose from your last meal for energy. Once this glucose is used up, it starts to tap into its stored glucose, known as glycogen, primarily located in the liver.
The Glycogen Depletion Phase (12-16 hours): After approximately 12 hours, your body's glycogen stores become significantly depleted. At this point, the 'metabolic switch' occurs, and your body transitions from burning glucose to burning stored fat for fuel. This is the stage where many of the key benefits of intermittent fasting begin to activate.
The Ketosis and Autophagy Phase (16-24+ hours): Beyond 16 hours, your body enters a more pronounced state of ketosis, where it breaks down fatty acids into ketone bodies to use as energy. Longer fasts, typically 24 hours or more, also trigger a cellular repair process called autophagy, where cells clean out and recycle damaged components.

Common Intermittent Fasting Schedules and Their Duration

There is no single answer to how many hours of not eating is considered a fast, as the optimal duration depends on your health goals and lifestyle. Several popular protocols exist:

The 12:12 Method

This is the most beginner-friendly fasting method, requiring you to fast for 12 hours and eat within a 12-hour window. A typical schedule might involve eating between 7 a.m. and 7 p.m. This is often achievable simply by not snacking after dinner and skipping breakfast until the next morning. It allows for rest and reset of the digestive system and can support weight management by reducing the eating window.

The 16:8 Method

The 16:8 method is one of the most common and involves fasting for 16 hours and eating during an 8-hour window. A typical schedule might involve eating from noon to 8 p.m. and fasting the rest of the day. This schedule is effective for weight loss, as the longer fasting window promotes more significant fat burning and can lead to a natural reduction in overall calorie intake.

The 5:2 Diet

This approach involves eating normally for five days a week and significantly restricting calories on two non-consecutive days, often to around 500-600 calories. This method can also be effective for weight management by creating a weekly calorie deficit without requiring daily fasting.

The Eat-Stop-Eat Method

This involves a full 24-hour fast, once or twice a week. For example, a person might fast from dinner one evening to dinner the next. While it can be more challenging and lead to side effects like fatigue and headaches, it can also lead to more profound metabolic benefits like a significant boost in human growth hormone and cellular repair.

Comparison of Common Fasting Methods


Aspect	12:12 Method	16:8 Method	24-Hour Fast (Eat-Stop-Eat)
Fasting Duration	12 hours	16 hours	24 hours
Eating Window	12 hours	8 hours	Normal eating on non-fast days
Typical Schedule	7 p.m. to 7 a.m. fast	8 p.m. to 12 p.m. fast	Dinner-to-dinner fast 1-2 times/week
Difficulty Level	Easiest for beginners	Moderate, most popular for results	Challenging, but manageable weekly
Primary Benefits	Digestive rest, weight management	Increased fat burning, weight loss	Enhanced autophagy, metabolism reset
Risks/Side Effects	Minimal	Hunger, irritability during adjustment	Fatigue, headaches, irritability
Suitability	Best for newcomers and general wellness	Effective for consistent weight loss	Best for those experienced with fasting

Conclusion: Which Fasting Duration is Right for You?

While a minimum of 12 hours of not eating is considered a fast in the scientific community, the ideal duration is personal. Beginners can start with the simple and sustainable 12:12 method, which offers digestive benefits and can aid in weight control. The 16:8 method is a popular choice for those seeking more pronounced weight loss and fat-burning effects. For more advanced fasters, a weekly 24-hour fast can maximize cellular repair processes like autophagy. Regardless of the method, it is crucial to stay hydrated with calorie-free beverages like water and black coffee during the fasting window. Always consult with a healthcare professional before starting any new fasting regimen, especially if you have pre-existing health conditions like diabetes. A balanced, nutrient-dense diet during eating periods is also key to long-term success. The best approach is the one you can adhere to consistently while fueling your body with healthy foods when you do eat. Learn more about the biology of fasting at the National Institutes of Health.

Frequently Asked Questions

After 12 hours without food, your body depletes its stored glucose (glycogen) and begins to shift its metabolic state to burn stored fat for energy.

No, black coffee typically does not break a fast as it contains a negligible amount of calories. However, adding milk, sugar, or cream will break a fast.

Yes, a 12-hour fast can be effective for weight management by naturally reducing the eating window, which can lead to consuming fewer calories overall. It is also an excellent starting point for beginners.

The 16:8 method involves fasting for 16 hours a day and consuming all meals and snacks within an 8-hour window. This is a popular schedule for promoting weight loss and other health benefits.

To break a fast gently, it is best to choose nutrient-dense foods like protein and healthy fats. High-carb or sugary foods can cause a blood sugar spike and are not recommended.

Many people practice daily intermittent fasting with methods like 12:12 or 16:8. However, longer, more restrictive fasts like the 24-hour method are typically done only once or twice a week.

Exercising during a fast is generally safe and can help enhance fat burning. However, it is important to listen to your body and stay hydrated, especially during longer fasts.