Understanding How Long Should You Go Without Eating When Intermittent Fasting?

October 20, 2025 •

4 min read

Over the past few decades, our eating patterns have shifted towards frequent snacking, potentially impacting metabolic health. When exploring an alternative eating pattern like intermittent fasting, a crucial consideration for beginners is: 'How long should you go without eating when intermittent fasting?'. The answer varies, depending on the specific method you choose and your personal health goals.

Quick Summary

Intermittent fasting involves alternating between eating and fasting periods, with the optimal duration depending on the method and individual needs. Popular schedules include daily time-restricted eating (like 16/8) and weekly fasting (like 5:2), which offer different benefits and challenges. Considerations like metabolic switching, personal experience, and health status are important for determining the right fasting length.

Key Points

Start with Shorter Fasts: Ease into intermittent fasting with a 12 or 14-hour fast, aligning with your sleep schedule, before trying longer durations.
Listen to Your Body: The ideal fasting duration is highly individual; pay close attention to your energy levels, mood, and hunger signals to adjust your schedule as needed.
Stay Hydrated: Drink plenty of calorie-free beverages, such as water, black coffee, and unsweetened tea, to help manage hunger and maintain hydration during fasting periods.
Prioritize Whole Foods: During your eating windows, focus on nutrient-dense foods like lean protein, fruits, vegetables, and whole grains to maximize the health benefits and prevent deficiencies.
Consider Your Goals: For weight loss, a 16/8 schedule is often effective and sustainable, while longer fasts may be explored for enhanced metabolic effects like autophagy.
Consult a Healthcare Provider: It is crucial to speak with a doctor before starting intermittent fasting, particularly if you have underlying health conditions like diabetes.

The Science Behind Intermittent Fasting

Intermittent fasting (IF) is an eating pattern that cycles between periods of voluntary fasting and non-fasting. Unlike traditional diets that focus on what to eat, IF is centered on when to eat. The physiological shift that occurs during fasting, known as "metabolic switching," is a core component of its benefits. After several hours without food, the body depletes its stores of glucose and begins burning fat for energy. This process can lead to reduced insulin resistance, improved metabolic health, and weight loss. However, the length of the fast is a critical factor in determining which metabolic processes are activated and for how long. For instance, longer fasts are associated with more pronounced metabolic changes, including the activation of cellular repair processes like autophagy.

Popular Intermittent Fasting Methods and Durations

There are several common intermittent fasting schedules, each with a different fasting duration. The best choice depends on your lifestyle, experience level, and health objectives. It's crucial to start with a milder schedule to allow your body to adapt.

12:12 Method: This is one of the most beginner-friendly approaches, involving a 12-hour fasting window and a 12-hour eating window. For example, you might stop eating at 8 p.m. and resume at 8 a.m. the next morning. It is a natural way to ease into fasting, as much of the fasting period occurs overnight.
16:8 Method: One of the most popular schedules, the 16:8 method involves fasting for 16 hours and eating during an 8-hour window. Many people skip breakfast to make this work, fitting their meals between, for example, 12 p.m. and 8 p.m.. This duration is long enough for most people to experience metabolic switching.
18:6 Method: A slightly more advanced version of time-restricted eating, the 18:6 method narrows the eating window to 6 hours. This can lead to a greater reduction in overall calorie intake and potentially enhanced fat burning.
5:2 Diet: This method involves eating normally for five days a week and significantly restricting calories (to 500-600) on two non-consecutive days. It’s a flexible approach that doesn't require daily fasting periods.
Eat-Stop-Eat (24-Hour Fast): This involves a complete 24-hour fast, usually done once or twice a week, from dinner to dinner. This is considered a more advanced technique and can cause side effects like fatigue and headaches, especially for beginners.
Alternate-Day Fasting (ADF): This involves alternating between a regular eating day and a fasting day, where you either eat no calories or a very reduced amount (around 500 calories). It can be difficult for many to sustain due to the intensity of the fasting days.

Factors to Consider When Choosing a Fasting Duration

The optimal length of a fast is not a one-size-fits-all metric. Several personal factors should influence your decision.

Experience Level: If you are new to fasting, starting with a shorter and more manageable fast, like 12 hours, is wise. This allows your body to gradually adapt without overwhelming your system with prolonged hunger. As you become more comfortable, you can slowly extend the fasting window.
Health Goals: Your primary motivation for fasting can help determine the best duration. For weight loss, many find the 16/8 method to be highly effective and sustainable. For cellular cleansing (autophagy) or more significant metabolic shifts, longer fasting windows (18+ hours) may be necessary, though more challenging.
Lifestyle and Routine: Your daily schedule plays a major role in how feasible a fasting method is. Consider when you naturally feel hungry and how fasting will fit around work, social commitments, and exercise. For example, aligning your fasting period with your sleeping hours often makes it easier to manage.
Medical Conditions: Certain health conditions, like diabetes, require careful consideration and medical supervision before attempting intermittent fasting. Long fasts can be dangerous for some individuals. Always consult a healthcare provider before beginning, especially if you take medication.

Safe Fasting Practices and Breaking a Fast

Regardless of the duration you choose, practicing intermittent fasting safely is paramount. Proper hydration is key, so drinking plenty of water, black coffee, and unsweetened tea is essential during fasting periods. During your eating windows, prioritize nutrient-dense whole foods, including fruits, vegetables, lean proteins, and healthy fats, to avoid nutrient deficiencies and support your body. Avoid the temptation to overindulge in processed foods and sugary snacks, as this can undermine the health benefits of fasting. When breaking a longer fast, it's advisable to do so with care, starting with easily digestible foods to avoid shocking your digestive system.

Comparison of Common Intermittent Fasting Schedules


Method	Fasting Duration	Eating Window	Suitability for Beginners	Notes
12:12	12 hours	12 hours	Excellent	Gentle and easy to start, often includes overnight sleep.
16:8	16 hours	8 hours	Good	Very popular and sustainable for many; often involves skipping breakfast.
18:6	18 hours	6 hours	Intermediate	More restrictive and intense; requires careful planning of meals.
5:2	2 days (restricted)	5 days (normal)	Good	Flexible weekly approach; less daily consistency required.
24-Hour Fast	24 hours	Varies	Advanced	Done 1-2 times weekly; can cause stronger side effects.
Alternate-Day	Every other day	Every other day	Advanced	Can lead to a 'feast or famine' mindset; requires medical supervision.

Conclusion: Personalizing Your Fasting Duration

The optimal fasting duration is not a fixed number but a personalized approach based on your body's needs and how you feel. For beginners, starting with shorter durations like 12 or 14 hours is a safe way to test the waters. As your body adapts, you can gradually increase the fasting window if you wish to explore methods like 16/8 or longer, weekly fasts. Always prioritize consuming nutrient-rich, whole foods during your eating periods to maximize health benefits and prevent nutritional deficits. Regular monitoring of your energy levels and overall well-being is essential for determining what works best for you. Most importantly, consult with a healthcare professional before starting any new fasting regimen, especially if you have pre-existing health conditions or are taking medication, to ensure it is done safely and effectively. For more information, the Johns Hopkins Medicine guide to intermittent fasting offers further insights.

Frequently Asked Questions

For a beginner, the 12:12 method is an excellent starting point. It involves fasting for 12 hours and eating within a 12-hour window, making it easy to incorporate, as much of the fasting occurs while sleeping.

For most people following time-restricted eating plans like 16/8, significant muscle loss is not a concern, especially when combined with resistance training. However, prolonged fasts over several days may negatively impact muscle mass.

Yes, zero-calorie beverages such as water, black coffee, and unsweetened tea are generally permitted during fasting periods. These can help manage hunger and keep you hydrated.

Fasting for too long, especially for several days, can be dangerous and lead to side effects like fatigue, headaches, irritability, and potentially prompt the body to store more fat later.

Daily time-restricted eating, such as the 16/8 method, is common and sustainable for many people. The appropriate frequency depends on your chosen method and individual needs.

When breaking a fast, prioritize nutrient-dense, whole foods. Good choices include lean proteins, fresh vegetables, healthy fats, fruits, and legumes. Avoid processed and sugary foods.

It can take 2 to 4 weeks for your body to adjust to intermittent fasting. Consistent adherence, combined with a healthy diet, is key for seeing benefits over time.