Choosing the Best Length of Time for Intermittent Fasting

October 20, 2025 •

6 min read

According to a 2023 meta-analysis of 99 clinical trials, various forms of intermittent fasting (IF) can lead to significant weight loss and improvements in cardiometabolic health. When determining what is the best length of time for intermittent fasting, the key is to understand that there is no one-size-fits-all answer, as the optimal duration depends on your goals, health, and ability to adhere to the schedule.

Quick Summary

Intermittent fasting involves alternating between periods of eating and fasting to achieve various health benefits, including weight loss and improved metabolic health. The ideal duration varies based on individual factors like lifestyle and experience level, with popular methods ranging from daily time-restricted eating to weekly 24-hour or alternate-day fasts. Beginners are often advised to start with shorter fasts and gradually increase duration for better adherence and adaptation.

Key Points

Start Slow: Begin with a 12 to 14-hour fasting window to allow your body to adapt before increasing the duration.
Choose Based on Goals: Shorter daily fasts (e.g., 16:8) are often ideal for weight management and beginners, while longer fasts (e.g., 5:2) can produce faster results but are harder to sustain.
Prioritize Consistency: The best fasting length is one you can stick with long-term, so find a schedule that fits your lifestyle to maximize adherence.
Eat Nutritious Foods: The benefits of IF are amplified by eating a healthy, balanced diet of whole foods during your eating window.
Stay Hydrated: Drink plenty of water and calorie-free fluids during your fasting period to manage hunger and prevent dehydration.
Consult a Professional: Always talk to a doctor before beginning any new fasting regimen, especially if you have pre-existing health conditions.

Understanding the Principles of Intermittent Fasting

Intermittent fasting isn't about what you eat, but when you eat. The goal is to extend the time your body goes without food, triggering a metabolic switch. After hours without food, the body exhausts its readily available sugar (glucose) stores and begins to burn stored fat for energy, a state known as ketosis. The duration of your fast influences the depth and duration of this metabolic shift, affecting the specific benefits you may experience, from weight loss to cellular repair.

Popular Intermittent Fasting Schedules and Their Lengths

Different fasting schedules offer varying fasting lengths, each with its own pros and cons. Here's a breakdown of some of the most common methods:

12:12 Method: This simple time-restricted eating (TRE) involves a 12-hour eating window and a 12-hour fasting window. It's a gentle introduction to IF, often involving nothing more than finishing dinner earlier and having breakfast later. This is often recommended for beginners to ease into fasting.
16:8 Method: One of the most popular forms of TRE, this method restricts eating to an 8-hour window each day, followed by a 16-hour fast. Many people find it manageable, often by skipping breakfast and having their meals between noon and 8 p.m.. Studies suggest this can be effective for weight management and may improve blood sugar processing.
18:6 and 20:4 (Warrior Diet): These are more advanced forms of TRE with shorter eating windows. The 18:6 method has a 6-hour eating window, while the 20:4 (Warrior Diet) restricts eating to just 4 hours. These more intensive methods can accelerate results for experienced fasters but can also be more challenging to sustain.
5:2 Method: This schedule involves eating normally for five days a week and restricting calories to 500-600 on two non-consecutive days. Research has found that this method is comparable to traditional calorie restriction for weight loss and is easier for some people to stick with.
24-Hour Fast (Eat-Stop-Eat): This involves fasting for a full 24 hours, once or twice a week, from dinner to dinner or lunch to lunch. This can be more challenging and may cause side effects like fatigue and headaches, especially for newcomers.

Factors Influencing Your Ideal Fasting Length

Choosing the right fasting duration is a personalized process. Your decision should be based on your health goals, current fitness level, and how your body responds.

Your Goals

For Weight Loss: Shorter, daily time-restricted schedules like the 16:8 method are often sufficient for creating a calorie deficit and managing weight over time. For more aggressive weight loss, longer fasts like the 5:2 diet or 24-hour fasts can be more effective, but may be harder to maintain.
For Metabolic Health: Improved insulin sensitivity and blood sugar regulation can be achieved with consistent daily fasts of at least 14-16 hours. For these benefits, consistency is often more important than extreme duration.
For Autophagy: This cellular "cleanup" process is activated during fasting, but its optimal duration is still being researched. Extended fasts of 24 hours or more are thought to significantly boost this process, but shorter, regular fasts can also be effective.

Your Body and Lifestyle

Beginners: Starting with a shorter, more manageable fast, such as the 12:12 or 14:10 method, is highly recommended. This allows your body to adapt gradually and helps prevent common side effects like headaches and fatigue.
Sustaining the Fast: Your ideal length is one you can stick with long-term. If a 16:8 schedule fits your social life and work routine better than a more restrictive 20:4 plan, it will be more sustainable and, therefore, more effective in the long run.
Hydration and Nutrition: The quality of food during your eating window is crucial. Regardless of the fast's length, it is vital to consume nutrient-dense foods to prevent deficiencies. Hydration with water and calorie-free drinks is also critical during all fasts.

Comparing Different Intermittent Fasting Schedules


Feature	16:8 (Time-Restricted)	5:2 (Modified Fasting)	24-Hour Fast (Eat-Stop-Eat)
Fasting Frequency	Daily	2 non-consecutive days per week	1-2 times per week
Ease of Adoption	High (good for beginners)	Medium	Low (more challenging)
Flexibility	Good (can shift window)	High (can choose any 2 days)	Medium (requires planning)
Main Benefit	Sustainable weight management, metabolic switch	Weight loss from calorie deficit	Advanced metabolic benefits, autophagy
Side Effects	Hunger, irritability (initial)	Hunger, fatigue on fasting days	Fatigue, headaches, dizziness
Long-Term Adherence	High	Medium to High	Low to Medium

A Final Word on Safety

While many people can safely practice IF, it's not for everyone. Longer and more intense fasts carry higher risks, including severe hunger, dizziness, and fatigue. In 2024, an observational study presented at an American Heart Association conference suggested a potential link between an 8-hour time-restricted eating schedule and a higher risk of cardiovascular death, especially in those with pre-existing conditions. However, this study did not prove causation, and more research is needed. Always consult with a healthcare provider before starting any new diet, especially if you have underlying health conditions, are pregnant, or are taking medication.

Conclusion

There is no single "best" length of time for intermittent fasting; the ideal duration is deeply personal. For most beginners, a shorter, daily time-restricted eating window, such as 12-16 hours, offers a safe and effective starting point for experiencing health benefits like weight management and improved blood sugar control. As you become more accustomed to the practice, you can explore longer, more intensive fasting protocols like the 5:2 diet or 24-hour fasts, if they align with your goals and health status. The most important factor for success is finding a sustainable schedule that integrates well with your lifestyle and focusing on a nutrient-dense diet during your eating windows. By listening to your body and consulting a professional, you can harness the benefits of intermittent fasting safely and effectively.

What are the Different Methods of Intermittent Fasting?

There are several methods of intermittent fasting, each involving a different pattern of eating and fasting periods. For example, the 16:8 method involves a daily 16-hour fast with an 8-hour eating window, while the 5:2 diet consists of two non-consecutive days of calorie restriction (500-600 calories) per week. Other options include alternate-day fasting and the more extreme 20:4 Warrior Diet.

How to Start Intermittent Fasting?

Start slowly and listen to your body. A 12-hour daily fast is a good entry point for beginners, as it naturally incorporates your overnight sleep. Gradually extend your fasting window by an hour or two as you adapt. During your fast, stay hydrated with water, black coffee, or herbal tea.

How to Deal With Hunger During the Fasting Window?

Managing hunger is a common challenge, especially when you first start. Staying busy can help distract you, while drinking plenty of water, black coffee, or green tea can help curb hunger pangs. Many people find that initial hunger and irritability subside as their bodies become accustomed to the new eating pattern.

What to Eat During the Eating Window?

During your eating window, focus on nutrient-dense, whole foods. Prioritize lean proteins, healthy fats, complex carbohydrates, and plenty of vegetables and fruits to ensure you get adequate vitamins, minerals, and fiber. Avoiding processed foods and excessive sugar is crucial for maximizing benefits.

Is Intermittent Fasting Safe for Everyone?

No, intermittent fasting is not suitable for everyone. It is not recommended for children, pregnant or breastfeeding women, individuals with a history of eating disorders, or people with Type 1 diabetes. Those with underlying medical conditions or taking certain medications should consult a doctor before starting.

What Are the Potential Side Effects of Intermittent Fasting?

Initial side effects can include hunger, fatigue, headaches, and irritability. Some may also experience dizziness, constipation, or trouble concentrating. These symptoms often decrease as the body adjusts, but prolonged or severe issues should be discussed with a doctor.

How Does Intermittent Fasting Affect Weight Loss Long-Term?

While IF can be effective for short-term weight loss, long-term studies are more limited, and findings on sustained effectiveness vary. The success of long-term weight management with IF, as with any dietary plan, ultimately depends on consistency and adhering to a healthy, balanced diet during eating periods.

Frequently Asked Questions

For beginners, the 12:12 or 14:10 time-restricted eating methods are highly recommended. This involves fasting for 12 or 14 hours each day, with the majority of the fasting period occurring overnight. This allows your body to gently adapt to the new eating pattern.

For effective weight loss, a daily 16-hour fast (16:8 method) is often recommended and has shown positive results. Some people find more intense methods like the 5:2 diet or a 24-hour fast once or twice a week also effective for creating a calorie deficit.

Not necessarily. While longer fasts (e.g., 24+ hours) can significantly boost processes like autophagy, they are also harder to sustain and carry a higher risk of side effects like fatigue. For most general health benefits, such as metabolic health, consistent shorter fasts (14-16 hours) are sufficient and more sustainable.

During your fasting window, you can drink calorie-free beverages like water, black coffee, and unsweetened herbal tea. These will keep you hydrated without breaking your fast. Avoid adding sugar, cream, or milk, as they contain calories that will interrupt the fasted state.

If not done correctly, long fasting periods or insufficient protein intake during eating windows can lead to muscle loss. Combining IF with a balanced, high-protein diet and regular resistance training can help preserve muscle mass.

The frequency depends on your chosen method. Daily methods like 16:8 are performed every day, while others like the 5:2 diet require fasting only two days a week. The best approach is to find a routine that you can adhere to consistently.

Yes, it is common to experience headaches, fatigue, and hunger when you first start intermittent fasting, especially with longer fasts. These side effects often subside as your body adjusts, and staying well-hydrated can help.