How many hours of fasting is considered fasting?

December 30, 2025 •

4 min read

Scientific research indicates that for the body to transition into a fasted state, a minimum period of 12 hours without food is required. However, the specific duration considered a 'fast' can vary significantly depending on the individual's goals, methods, and physiological response.

Quick Summary

The minimum threshold for a fasted state is generally 12 hours, but different protocols require longer durations to trigger specific health benefits like increased fat burning or autophagy. Various factors influence the optimal fasting length.

Key Points

12-Hour Threshold: The body typically enters a fasted metabolic state after approximately 12 hours without food, transitioning from burning glucose to fat.
Longer Fasts, Greater Benefits: Extending the fasting window beyond 12 hours, such as with a 16:8 or 20:4 protocol, intensifies fat burning and can trigger cellular repair processes like autophagy.
Multiple Approaches: Different fasting methods, including daily time-restricted eating (e.g., 16:8) and weekly fasts (e.g., 5:2), vary in duration and intensity to suit different lifestyles.
Not for Everyone: Fasting is not recommended for pregnant or breastfeeding women, children, or individuals with a history of eating disorders or specific medical conditions without a doctor's supervision.
Start Simple: Beginners can start with a shorter, more manageable 12-hour fast, which can be easily achieved overnight, before attempting longer periods.
Consult a Professional: Always seek medical advice before starting a new fasting regimen, especially if you have underlying health issues.

Understanding the Physiology of Fasting

For most people, the body enters a true metabolic fasted state approximately 12 hours after the last meal. Before this point, the body is in a 'fed' or 'post-absorptive' state, digesting and absorbing nutrients from food. Once the body has depleted its glycogen stores—its primary source of energy from carbohydrates—it initiates a metabolic switch to begin burning stored fat for fuel instead. This critical switch is the foundation for many of the reported benefits of intermittent fasting, such as weight loss and improved insulin sensitivity.

Common Fasting Protocols and Their Duration

Different intermittent fasting protocols are defined by specific fasting windows. The following outlines the hours involved in some of the most popular methods:

12-Hour Fast (12:12): A simple and popular starting point for many beginners. This method involves fasting for 12 hours each day and confining eating to the remaining 12-hour window. It is often achieved by simply extending the natural overnight fast, for example, stopping eating at 7 p.m. and not eating again until 7 a.m. the next morning. This can be an effective and sustainable way to begin improving metabolic health.
16-Hour Fast (16:8): Known as the Leangains protocol, this method involves a 16-hour fast followed by an 8-hour eating window. It is often achieved by skipping breakfast and having the first meal around noon. Many find this schedule manageable as it aligns with existing sleeping patterns. This longer period without food allows for more significant metabolic switching and fat burning compared to a 12-hour fast.
Weekly 24-Hour Fasts (Eat-Stop-Eat): This involves fasting completely for one or two full 24-hour periods each week. The fast typically runs from one meal to the same meal the next day, for example, from dinner on Monday to dinner on Tuesday. During the fast, only water, tea, or other calorie-free beverages are consumed. This method offers a more intense approach to calorie restriction.
5:2 Diet: With this modified fasting approach, an individual eats a normal, healthy diet for five days of the week and restricts calories significantly on two non-consecutive days. On these two days, calorie intake is limited to around 500 for women and 600 for men. While not a true 'no food' fast, it is recognized as a form of intermittent fasting.
The Warrior Diet: This is a more extreme form of time-restricted eating, involving a 20-hour fast followed by a 4-hour eating window. It is recommended for more experienced fasters and focuses on consuming one large meal at night. This method is associated with a deeper fasted state and potentially increased autophagy.

The Timing of Cellular Benefits

The duration of fasting has a direct impact on the body's cellular processes. For instance, autophagy—the process where the body cleans out damaged cells and regenerates new ones—is thought to be activated at a higher rate during longer fasts. While signs of autophagy can be measured in humans in less than 24 hours, animal studies suggest the process intensifies significantly after 36 to 72 hours. This means that while a 12-hour fast can provide solid metabolic benefits, longer fasts may be required to maximize certain cellular repair functions. It is crucial to remember that prolonged fasting should only be done under medical supervision to ensure safety.

Comparison of Common Fasting Methods


Fasting Method	Fasting Period	Eating Window	Key Features & Benefits	Best For
12:12	12 hours daily	12 hours daily	Easy for beginners, aligns with natural sleep cycle, promotes metabolic switch.	First-timers or those looking for gentle metabolic improvement.
16:8	16 hours daily	8 hours daily	Popular and sustainable, more aggressive fat-burning than 12:12, improves insulin sensitivity.	Individuals with some fasting experience aiming for weight loss.
Eat-Stop-Eat	24 hours (1-2 times weekly)	24 hours (2-3 times weekly)	Intense calorie reduction, effective for weight loss, can be challenging.	Experienced fasters looking for a structured, non-daily approach.
5:2 Diet	2 days (modified fast)	5 days (normal eating)	Flexible and less restrictive on most days, modest weight loss benefits.	Those who struggle with daily time-restricted eating.
Warrior Diet	20 hours daily	4 hours daily	Extreme approach, potential for high autophagy benefits, very restrictive.	Advanced fasters seeking a challenge or maximum benefits.

Who Should Not Fast or Fast with Caution

While fasting offers benefits for many, it is not suitable for everyone. Certain individuals should avoid fasting or proceed only under the guidance of a healthcare professional. This includes pregnant or breastfeeding women, children and adolescents under 18, people with a history of eating disorders, and those with underlying health conditions like diabetes. It is essential to consult a doctor, especially if you are taking medication, to ensure any fasting plan is safe for your specific health needs.

Conclusion: How many hours of fasting is considered fasting? It Depends on Your Goals

In summary, the number of hours considered a fast is not a single, fixed number but a range that depends on the specific protocol and desired health outcome. The minimum threshold for metabolic benefits is generally accepted as 12 hours, a period that initiates the metabolic switch from burning glucose to fat. For more advanced benefits like increased fat-burning and cellular repair (autophagy), longer periods of 16 hours or more are typically required. By understanding the different fasting durations and their effects on the body, individuals can make an informed decision to find a sustainable approach that aligns with their personal health and wellness goals. Always prioritize safety by consulting a healthcare professional before making any significant dietary changes. You can find additional resources at Johns Hopkins Medicine.

Frequently Asked Questions

In a scientific context, a period of at least 12 hours without food is generally considered a fast, as this is when the body typically exhausts its stored glucose and switches to burning fat for energy.

During a 16-hour fast, the body continues to burn fat for energy after the initial 12-hour mark, leading to benefits such as increased fat loss and improved insulin sensitivity.

A 24-hour fast, like the 'Eat-Stop-Eat' method, can lead to a greater total calorie reduction and potentially deeper metabolic effects, but it is also more challenging and may cause side effects like fatigue. The longer fast is not necessarily 'better' but aims for different outcomes.

During a fast, you can typically drink calorie-free beverages such as water, black coffee, and plain tea to stay hydrated without breaking the fast. Caloric drinks, including those with sugar or milk, are not allowed.

Yes, the hours you spend sleeping are counted towards your total fasting time. For instance, a 12-hour overnight fast is an easy way for many people to incorporate fasting into their daily routine.

The 'metabolic switch' is the point during a fast when the body transitions from using glucose (sugar) as its primary fuel source to burning stored fat for energy. This process usually begins after about 12 hours without food.

Adapting to a new fasting routine can take two to four weeks. During this adjustment period, it is common to experience hunger, irritability, or low energy levels, but these effects often subside as the body adapts.