How many hours of fasting is good per day for health?

January 8, 2026 •

5 min read

According to Johns Hopkins neuroscientist Mark Mattson, our bodies evolved to go without food for many hours, which is a key concept behind intermittent fasting. Fasting for an optimal number of hours daily can trigger important metabolic shifts, but the ideal duration depends on individual health goals and lifestyle.

Quick Summary

Different fasting durations offer varying health benefits. For beginners, a 12-hour fast is a good starting point, while 16 hours is often considered optimal for metabolic health and weight loss. Longer fasts of 18+ hours can induce more significant cellular repair, but must be approached with caution and medical guidance. Personalizing your schedule is crucial for sustainability and success.

Key Points

Start Slow: Beginners should begin with a 12-hour fast, which is easy to manage and provides initial metabolic benefits.
16:8 is Optimal: A 16-hour daily fast is considered optimal for most individuals seeking weight loss and enhanced fat burning.
Cellular Repair Starts at 12-14 Hours: The process of autophagy, or cellular cleanup, is triggered after 12-14 hours of fasting and intensifies with longer durations.
Longer Fasts Require Caution: Daily fasts of 18 or more hours are more intense and should only be undertaken with professional medical supervision.
Listen to Your Body: Pay attention to hunger signals and side effects, and adjust your fasting schedule as needed for sustainability.
Prioritize Nutrition: During your eating window, focus on nutrient-dense, whole foods to maximize health benefits and prevent nutrient deficiencies.
Stay Hydrated: Always drink plenty of water and calorie-free beverages during fasting periods to prevent dehydration and manage hunger.

The Science Behind Daily Fasting

Intermittent fasting (IF) is an eating pattern that cycles between periods of eating and voluntary fasting. The core principle is allowing insulin levels to drop, prompting your body to switch from burning glucose (sugar) for energy to using stored fat. This metabolic switch is a key driver behind the health benefits associated with fasting. The time it takes to achieve this shift, and the subsequent processes that follow, depend largely on the duration of your daily fast.

The 12-Hour Fast: An Accessible Starting Point

A 12-hour daily fast is one of the most manageable and popular approaches for beginners. Many people already do this naturally by including their overnight sleep in the fasting window. For instance, finishing dinner by 7 p.m. and not eating again until 7 a.m. the next morning meets this criteria.

Easy to Adopt: This schedule is a simple and gentle way to introduce your body to the concept of time-restricted eating. It can also help curb late-night snacking.
Initial Metabolic Benefits: Research suggests that fasting for at least 12 hours can cause the body to use stored fat for energy. This initiates the metabolic switch and offers early improvements in blood sugar regulation.
Sustainability: Since it aligns with most natural sleep-wake cycles, this method is highly sustainable for long-term practice.

The 16:8 Method: Optimal for Weight Loss and Fat Burning

The 16:8 method is arguably the most common and widely studied form of intermittent fasting, involving an 8-hour eating window and a 16-hour fast. Extending the fasting period to 16 hours allows for more significant fat-burning and deeper metabolic effects.

Greater Fat Burning: During a 16-hour fast, your body has more time to deplete its glucose stores and rely on fat for fuel. Studies have shown that 16-hour fasts are more effective for fat loss than 12-hour fasts, especially when combined with a balanced diet.
Improved Insulin Sensitivity: A longer fasting period allows insulin levels to drop further, which can significantly improve insulin sensitivity and reduce the risk of type 2 diabetes.
Cellular Repair (Autophagy): As the fast extends beyond 12-14 hours, your body begins the process of autophagy, a cellular cleanup where damaged and old cells are recycled.

Extended Daily Fasts: 18+ Hours

While less common for daily practice, some fasting schedules involve fasting for 18 to 20 hours (e.g., the Warrior Diet) or even 24 hours (Eat-Stop-Eat). These durations intensify the metabolic effects but are also more challenging and require more caution.

Increased Autophagy: The cellular repair process intensifies significantly after 18 hours, with more research suggesting peak brightness between 36 and 72 hours.
Accelerated Fat Adaptation: Extended fasts force the body into a deeper state of ketosis, where it relies heavily on ketones produced from fat for energy, including brain fuel.
Not for Everyone: These longer, daily fasts can be difficult to sustain and are not recommended for beginners. It's crucial to listen to your body and consult a healthcare professional before attempting these more extreme forms of intermittent fasting.

Fasting Schedule Comparison Table


Feature	12:12 Fast	16:8 Fast	20:4 (Warrior Diet)	Alternate Day Fast (ADF)
Fast Duration	12 hours	16 hours	20 hours	~36 hours (every other day)
Eating Window	12 hours	8 hours	4 hours	~12 hours (on eating days)
Main Goal	Metabolic regulation, beginner-friendly	Weight loss, enhanced fat burning	Advanced fat loss, intense autophagy	Rapid weight loss, metabolic reset
Beginner Suitability	Excellent	Good	Not Recommended	Not Recommended
Fat Burning	Initiates metabolic switch	Significant increase	Maximized	Very high on fasting day
Cellular Repair (Autophagy)	Mildly triggered	Triggered and extended	Intensified	Peaks at 36+ hours
Sustainability	Very High	High	Low	Low
Caution	Minimal	Low	High (risk of nutrient deficiency)	Very High (requires medical supervision)

How to Choose the Right Fasting Duration

Selecting the right fasting duration is a personal journey that should align with your health goals, lifestyle, and how your body responds. Here's how to decide:

Assess Your Goals: If your goal is general metabolic health, improved digestion, or a gentle introduction to fasting, start with a 12-hour fast. For more pronounced weight loss and fat burning, the 16:8 method is a well-supported option. For experienced fasters looking to maximize benefits like autophagy, longer fasts might be explored under professional guidance.
Start Slowly and Adjust: The most sustainable approach is to start with a comfortable fasting window, like 12 hours, and gradually increase it as your body adapts. Many people increase their fast by an hour or two per week until they reach their desired duration.
Listen to Your Body: Pay close attention to how you feel. Common side effects like hunger, headaches, or irritability usually subside as your body adjusts. However, persistent symptoms or severe discomfort indicate you may need to adjust your fasting window or consult a healthcare provider.
Prioritize Nutrients: Regardless of your fasting duration, focus on consuming nutrient-dense, whole foods during your eating window. Calorie-free hydration, such as water, black coffee, or unsweetened tea, is crucial during the fasting period to help manage hunger and stay hydrated.

Considerations and Safety Precautions

Intermittent fasting is not suitable for everyone. Certain individuals should avoid fasting or seek medical advice before starting, including pregnant or breastfeeding women, children, and people with a history of eating disorders or specific health conditions like diabetes.

Furthermore, recent observational studies have raised concerns about very long daily time-restricted eating and heart health, though more research is needed to understand the correlation. Always prioritize balanced nutrition and consult a healthcare professional to ensure any fasting regimen is safe for you.

Conclusion

Ultimately, there is no single best answer to how many hours of fasting is good per day. The optimal duration is a personal choice, ranging from an easily manageable 12-hour fast for beginners to a more advanced 16-hour fast for weight loss and deeper metabolic benefits. Starting slow, listening to your body, and prioritizing nutrient-dense foods are the most important steps toward a healthy and sustainable intermittent fasting practice. By finding the right balance that suits your body and lifestyle, you can reap the numerous potential rewards of this eating pattern.

For more expert insights on fasting and metabolic health, consider exploring resources from the National Institutes of Health (NIH), which has extensively covered this topic through articles like To Fast or Not to Fast.

Frequently Asked Questions

The 12:12 intermittent fasting method is recommended for beginners. It involves fasting for 12 hours, typically overnight, and eating during a 12-hour window. This is a gentle introduction to time-restricted eating and is relatively easy to adhere to.

After about 10-12 hours of fasting, your body begins to deplete its glucose stores and starts using stored fat for energy. Extending your fast beyond this period, such as with a 16-hour fast, increases the duration of fat-burning.

For individuals seeking more significant weight loss and enhanced metabolic benefits, a 16-hour fast is generally considered more effective than a 12-hour fast. The longer duration allows for a more prolonged metabolic switch to fat burning and triggers cellular repair processes like autophagy.

Yes, you can drink calorie-free beverages like black coffee, unsweetened tea, and water during your fasting window. These liquids do not break your fast and can help manage hunger and maintain hydration.

Yes, it is possible to fast for too long, especially if done daily without professional guidance. Fasting for more than 20 hours daily can be difficult to sustain and may increase the risk of overeating during the feeding window or causing nutrient deficiencies. Always listen to your body and consult a healthcare provider.

Some studies have shown a potential link between intermittent fasting and lean muscle mass loss, especially with more restrictive protocols. However, other research suggests that fasting can promote fat loss while preserving muscle mass. Regular exercise and sufficient protein intake during eating periods are crucial for muscle maintenance.

Intermittent fasting is not safe for everyone. People who are pregnant or breastfeeding, children and teens under 18, and those with a history of eating disorders should avoid it. Additionally, individuals with diabetes or other chronic conditions should consult a doctor before starting any fasting regimen.