How long do you need to not eat to be fasting?

November 27, 2025 •

3 min read

According to nutrition experts, a minimum of 12 hours of not eating is required for the body to enter a fasted state. During this time, your body begins to shift from burning glucose for energy to burning stored fat. Understanding how long do you need to not eat to be fasting depends on your specific health goals.

Quick Summary

The duration of a fast varies depending on the desired outcome, with 12 hours being the minimum for basic metabolic changes. Benefits like increased fat burning and cellular repair occur with longer fasts, such as the popular 16-hour or extended 24-hour schedules. Proper hydration and choosing the right method for your lifestyle are key to success.

Key Points

Minimum 12 Hours: A 12-hour overnight fast is the minimum time needed for your body to transition from burning glucose to stored fat, an excellent starting point for beginners.
16-Hour Fasting: The popular 16:8 method extends the fast to 16 hours, promoting greater fat burning and is a common next step for intermittent fasting.
Autophagy Requires Longer Fasts: For cellular repair processes like autophagy, most evidence suggests fasting for at least 16 to 18 hours, with more significant effects occurring after 24 to 72 hours.
Stay Hydrated: Regardless of the fasting duration, drinking water, black coffee, or herbal tea is essential to stay hydrated and manage hunger.
Listen to Your Body: Side effects like fatigue, dizziness, and irritability can occur, especially with longer fasts. It is important to start gradually and stop if you feel unwell.
Consult a Professional: Always talk to a doctor before starting a new fasting routine, especially if you have health concerns or are on medication.

Understanding the Fasted State

For many, the term fasting conjures up images of extended periods without food, but the science behind it is more nuanced. A fast technically begins when your body is no longer actively digesting and absorbing food, which typically starts hours after your last meal. The real metabolic shift, however, begins when the body depletes its readily available glucose stores and switches to using stored body fat for energy.

The 12-Hour Fast: The Starting Point

A 12-hour fast is one of the most accessible forms of intermittent fasting and is often the natural first step for beginners. It's a method many people already do overnight without realizing it. For example, if you finish dinner by 7 p.m. and don't eat again until 7 a.m. the next day, you've completed a 12-hour fast. This approach allows your body to rest and reach a baseline glucose level, signaling the transition to a fat-burning state. For metabolic health, aiming for at least 12 hours between meals regularly can be beneficial.

Moving to 16 Hours and Beyond

For those seeking greater fat-burning potential and other health benefits, extending the fasting window is the next step. The 16:8 method is one of the most popular intermittent fasting protocols, involving fasting for 16 hours and restricting eating to an 8-hour window. For example, finishing dinner by 8 p.m. and having your first meal the next day at noon fulfills this requirement. This longer fasting period allows more time for the body to utilize its fat stores for energy.

The 24-Hour Fast: Cellular Repair

A 24-hour fast, often referred to as the 'Eat-Stop-Eat' method, is a more intensive approach where you fast completely for one or two non-consecutive days a week. During this time, the body's glycogen stores are fully depleted, and it significantly increases the process of autophagy, where cells break down and recycle damaged components. While challenging, a 24-hour fast can offer significant benefits for cellular repair and overall metabolic health. It's important to drink plenty of water and other calorie-free beverages during a 24-hour fast to stay hydrated.

Extended Fasting (48+ Hours)

Prolonged fasts, lasting 48 hours or more, should not be attempted without medical supervision, especially for individuals with underlying health conditions. Fasting for 48 to 72 hours can trigger deep autophagy and other profound metabolic shifts. However, the risk of side effects like dehydration and electrolyte imbalances increases significantly with extended durations. These longer fasts are not suitable for beginners and are best reserved for those with specific, medically-cleared health goals.

Comparison of Fasting Durations


Fasting Duration	Target Metabolic State	Key Benefits	Best For	Potential Downsides
12-14 Hours	Transition to fat burning.	Easy for beginners, improved digestion, blood sugar stabilization.	Beginners, those seeking moderate health improvements.	Modest results compared to longer fasts.
16-18 Hours	Increased fat oxidation, entry into ketosis.	Greater weight loss, enhanced fat burning, potential autophagy initiation.	Experienced intermittent fasters, weight management.	Can be harder to stick with initially, may cause hunger.
24 Hours	Full glycogen depletion, significant autophagy.	Deeper cellular repair, reduced inflammation, metabolic reset.	Experienced fasters, weekly or bi-weekly practice.	Intense hunger, headaches, fatigue.
48-72 Hours	Deep autophagy, full ketosis, significant metabolic shifts.	Profound cellular renewal, enhanced insulin sensitivity.	Experts only, requires medical supervision.	High risk of side effects, potential electrolyte imbalance.

Fasting Techniques and Considerations

Beyond the duration, successful fasting relies on strategy and listening to your body. Starting slowly with an achievable goal, like a 12-hour overnight fast, is the best approach. Hydration is paramount throughout any fast, especially longer ones. You can also try modified fasts, like the 5:2 diet, which involves restricting calories on certain days rather than a complete food abstention. The ultimate goal is sustainability; the best fasting regimen is one you can maintain long-term to achieve your health goals. For further reading on the science behind fasting, the National Institutes of Health provides extensive research.

Conclusion

To determine how long do you need to not eat to be fasting, consider your individual health goals and experience level. While 12 hours is enough to initiate basic metabolic changes, longer durations offer more profound benefits, such as enhanced fat burning and cellular repair through autophagy. Beginners should start with shorter fasts and prioritize hydration, while more experienced individuals can explore longer protocols with proper planning and medical advice. It's crucial to consult a healthcare provider before beginning any new fasting regimen, particularly longer ones or if you have pre-existing health conditions.

Frequently Asked Questions

A fast generally begins when your body is no longer digesting food. In the scientific community, at least 12 hours of not eating is considered the start of a true fasted state.

During a 12-hour fast, your body depletes its immediate glucose stores and begins the metabolic shift towards burning stored body fat for energy. This is a great starting point for weight management and beginners.

While the exact timing can vary, most research suggests that significant autophagy activity begins after 16 to 18 hours of fasting, with the peak occurring between 36 and 72 hours.

For healthy adults, a 24-hour fast (e.g., the Eat-Stop-Eat method) is generally considered safe. However, individuals with health issues or taking certain medications should consult a doctor first.

During your fasting window, you can consume calorie-free beverages such as water, black coffee, or unsweetened herbal teas. These help with hydration and can curb hunger.

Common mistakes include starting with overly long fasts, not staying properly hydrated, and overeating during the eating window to compensate for lost calories. Beginners should start slow and listen to their body.

The 16:8 method involves fasting for 16 hours and eating during an 8-hour window each day. It is one of the most popular and flexible intermittent fasting regimens.