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How Long Should You Fast For? Determine Your Optimal Fasting Time

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When people turn to popular diets to lose weight, such as low-carbohydrate diets, like the Atkins diet or the ketogenic diet, or low-fat diets, which were particularly popular in the 1990s, there is still another common approach to losing weight.

Another very popular approach to dieting and weight loss is intermittent fasting, which involves restricting the time in which you eat your food (time-restricted intermittent fasting), or alternating days between eating normally and either doing a 24-hour water fast or significantly reducing your caloric intake (such as the 5:2 diet).

But how long should you fast for? What is the optimal fasting time? Should you do 18/6 intermittent fasting, 24-hour water fasts, longer fasts like 3-day water fasts, or something like 20/4 intermittent fasting?

In this article, we will answer your question, how long should you fast for, by discussing how to determine your optimal fasting length.

We will cover: 

  • How Long Should You Fast For? How to Determine Your Optimal Fasting Time
  • How Long Do Most People Fast?
  • Is Fasting the Best Way to Lose Weight?

Let’s jump in!

An alarm clock with a yellow background.

How Long Should You Fast For? How to Determine Your Optimal Fasting Time

Once you decide that you want to try an intermittent fasting diet, one of the most difficult decisions becomes determining your optimal fasting time or length. In other words, how long should you fast for?

As with most things related to diet and health, the optimal fasting time for each person is individualized rather than there being some sort of universal best amount of time to fast for everyone.

We all have somewhat unique biochemistries based on our genes, sex, body size, and the way our individual metabolism functions. Therefore, how long you should fast can vary from one individual to the next.

Some people do very well fasting for 20 to 24 hours a day, whereas other people are best served with more moderate fasting lengths of 14 to 18 hours or so.

A closeup of a clock.

The benefits of fasting will be somewhat dependent on how long you fast for.

However, even if there are greater potential health and weight loss benefits of fasting to be gleaned by extending your fast for a longer period of time, if the prolonged fasting leads you to feel lightheaded, weak, hungry, irritable, or low energy, or results in you rebounding with compensatory overeating, your own personal optimal fasting length may be shorter.

The best strategy to determine how long you should fast for is really a matter of trial and error, experimenting with different fasting durations, noting how you feel and how your body responds and then deciding where the “sweet spot“ for optimal fasting length seems to fall for you and your body in your life.

It’s also worth mentioning that the best fasting length for you can change over time and between different situations. 

For example, if you are following an intermittent fasting diet, you might have an optimal fasting time on days when you do not work out that is longer than your optimal fasting time on days that you exercise.

A teal clock and a salad.

You will perform better in your workouts and recover more effectively if you are properly fueling beforehand and afterward.

There can be additional factors that affect the optimal amount of time you should fast from day to day as well.

For example, if you slept really poorly, feel like you were coming down with an illness of some type, have been under a lot of stress, or are simultaneously following a diet alongside intermittent fasting, such as carb cycling, that involves significantly altering your macronutrient ratios, your ideal fasting length may be shorter.

How Long Do Most People Fast?

Among the many intermittent fasting schedules and intermittent fasting regimens that involve time restricted-eating, it seems that the most popular approach to intermittent fasting is the 16/8 intermittent fasting diet, which involves fasting for 16 hours per day and allowing for an eight-hour eating window.

For example, if you are going to be doing a 16-hour daily fast, you might begin your fast each night after finishing your final meal at 7 PM. You would then fast overnight until the next morning at 11 AM. Then, you could break your fast with your first meal and eat all of your caloric intake between the hours of 11 AM and 6 PM. 

A post-it that says 16 hour fasting.

Of course, you can shift the 16-hour fasting period and eating window in either direction, but most people following the 16/8 intermittent fasting diet eat from 10 AM to 6 PM, 11 AM to 7 PM, or noon to 8 PM.

The benefit of this intermittent fasting schedule is that most people seem to tolerate the 16-hour fast fairly well and can follow the diet in a sustainable way without being massively uncomfortable or hungry for large portions of the day.

Fasting for 16 hours per day can be an optimal fasting length for people who are physically active and work out most days of the week because the eight-hour eating window still allows for plenty of time to get in a workout during that period of time when you can fuel your body before and after your workout. 

This fasting time also works well for people with faster metabolisms or who tend to feel lethargic and very hungry when fasting for longer periods of time.

However, there are potentially more benefits to be gained by fasting for longer than 16 hours per day.

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Some people find that an 18-hour fast is the optimal fasting length because it strikes the balance between maximizing the health benefits of fasting without crossing over into being too extreme in that it’s neither unsustainably uncomfortable from an energy or appetite standpoint nor leads to extreme hunger and/or binge eating once your fasting window opens up.

Ultimately, how long you should fast per day really depends on your goals.

If you are primarily doing intermittent fasting to lose weight, you should find the fasting length that helps you best control your total caloric intake without tightening your appetite such that you overeat once it’s finally time to eat.

In other words, people use intermittent fasting diets to help set boundaries and restrict when they can eat so that they are not permitted to eat willy-nilly and mindlessly consume calories all day. 

While this is great in theory and often does lead to weight loss, if you are fasting too long per day such that you become so famished, you may overeat once it’s time to finally eat.

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This can lead to “undoing“ the caloric deficit you generated by fasting, essentially consuming the same number of calories, if not more, than you would have had you fasted for a shorter period of time or just eaten on a normal eating schedule.

For this reason, for people who are fasting for weight loss, the best fasting length is often more moderate (such as 16 hours) than for those who are fasting for health reasons (wherein it might be 20 or more hours).

On the other hand, many of the reported health benefits of fasting, particularly autophagy and a significant reduction in inflammation and oxidative stress, only come once you have fasted long enough.

Although evidence from research studies has been mixed, this may be anywhere from 18 to 36 hours or more, particularly if you are looking for significant autophagy to occur. Autophagy is the process of cellular cleanup.

Viable cellular components and proteins are recycled, while damaged and misfolded proteins, such as those linked to neurodegenerative diseases like Alzheimer’s, are destroyed, helping protect your body from such diseases and other sources of oxidative damage and premature aging.

A clock and utensils on a plate.

Is Fasting the Best Way to Lose Weight?

Generally speaking, the most effective approach to losing weight is to work one-on-one with a nutritionist or registered dietitian to come up with a meal plan that will best suit your individual needs. 

Unfortunately, however, many people do not have the luxury of affording this type of service or do not have access to a professional. Speaking with your general healthcare provider can also be a good first step.

It is highly recommended that if you are going to be doing any prolonged fasting over 24 hours, you should work with your doctor and obtain medical supervision during your fast or at least medical clearance beforehand.

Prolonged fasting is not necessarily safe or effective for everyone, and it can be associated with potential adverse health symptoms such as hypoglycemia, low blood pressure, dizziness, electrolyte imbalances, and muscle protein breakdown, among other potential risks.

For more information and complete guides for all fasting schedules, check out our database full of essential fasting information here.

A doctor holding a sign that says intermittent fasting.
Photo of author
Amber Sayer is a Fitness, Nutrition, and Wellness Writer and Editor, as well as a NASM-Certified Nutrition Coach and UESCA-certified running, endurance nutrition, and triathlon coach. She holds two Masters Degrees—one in Exercise Science and one in Prosthetics and Orthotics. As a Certified Personal Trainer and running coach for 12 years, Amber enjoys staying active and helping others do so as well. In her free time, she likes running, cycling, cooking, and tackling any type of puzzle.

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