We often get so fixated on the length of fasting hours that we overlook the importance of timing. That’s not to say that the duration of your fast isn’t crucial—it absolutely is. However, timing your fasting window correctly can actually have a significant impact on your IF journey, potentially making or breaking your overall experience.
It’s not as simple as just choosing any 16-hour window in which to fast. The ‘how’ and ‘what’ of your fasting windows can affect everything from your energy levels to your weight loss success.
Here’s what you need to know: the timing of your eating and fasting periods, synchronized with your body’s natural circadian rhythm, can be a game-changer for achieving your health goals.
In fact, mastering your fasting windows can be the difference between a productive, energizing IF routine and one that leaves you feeling drained and unsatisfied.
Read on to find out how strategically setting your fasting windows can enhance your IF experience.
What Are Fasting Windows?
Fasting windows are the time between your last meal of the day and your first meal of the next day. It’s the period during which your body is in a fasted state, burning stored glucose and fat for energy instead of relying on food consumption.
For example, if you follow the popular 16/8 method, your fasting window is 16 hours long and your eating window is eight hours long. If you’re doing alternate day fasting (ADF), you will alternate between eating normally on one day and fasting for the entire day the following day. In this case, your fasting window would be 24 hours.
During the fasting period, you can typically only consume water, black coffee, or tea without any additives. Some versions allow a small amount of food to be consumed during the fasting period.
What Is a Good Fasting Window?
In our opinion, a good fasting window fulfills the following criteria:
Is Long Enough for Your Body to Enter Ketosis
The longer your fasting window, the greater your body’s chances of entering ketosis—the state in which your body burns fat for energy instead of glucose. This is because it is believed to take an average of 16-22 hours for our bodies to burn through glycogen stores and reach a ketogenic state (7).
If your fasting window is too short (say, 10-12 hours), your body may not have enough time to deplete glycogen stores and start using fat for energy. As a result, you may not experience the full benefits of IF, such as improved insulin sensitivity and accelerated weight loss.
Syncs with Your Circadian Rhythm
The fasting window should align with your natural circadian rhythm, ideally confining eating periods to daytime hours.
Eating in sync with this natural cycle has been associated with better overall health outcomes. It promotes restful sleep, optimized digestion, and improved weight management (2).
At the same time, late-night eating (i.e., that which occurs outside of typical daytime hours) can interfere with your body’s natural processes, causing sleep disturbances and digestive issues (3). Furthermore, research has indicated that eating during the day is more metabolically efficient than eating at night.
Therefore, when setting your fasting windows, you should consider your daily routine and try to align them with daytime hours for maximum benefits.
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Provides Sufficient Time for Nourishment
The non-fasting period should allow enough time for you to consume the necessary nutrients and calories.
IF is not about calorie restriction, it’s about timing your meals strategically (6). Therefore, your eating period should be long enough to eat an adequate amount of food to meet the needs of your body.
Restricting your eating window too much may lead to undernourishment and leave you feeling lethargic or hungry throughout the day. At the same time, extending your eating window for too long may result in overeating and hinder your weight loss efforts.
The best intermittent fasting window for weight loss and other benefits is one that allows you to consume balanced, nutrient-dense meals within an appropriate time frame.
Accommodates Physical Activity
Combining your intermittent fasting schedule with exercise is a good way to accelerate weight loss and improve overall fitness (9). However, your fasting window should consider the timing of your workouts.
Exercising while in a fasted state may have benefits for fat burning and muscle building (4). However, it’s not suitable for everyone, and some people may experience negative side effects including dizziness or fatigue.
Therefore, if you plan on exercising during your fasting window, you have several options depending on your goals and preferences:
- Exercise during your fasted state, before breaking your fast. This may help burn more fat and improve metabolic flexibility (4).
- Break your fast with a small snack or meal, wait for 30 minutes to an hour, then exercise. This can provide you with enough energy to perform at your best.
- Exercise after breaking your fast with a full meal. Some people find that working out after eating is more comfortable and helps with performance.
Aligns with Your Sleep Cycle
Ideally, the majority of the fasting period should occur during your sleep time. This will allow your body to go through the natural fasted state, which is essential for various cellular repair processes and hormone regulation.
Moreover, avoiding large meals close to bedtime can improve sleep quality by reducing digestive issues (10).
So if you have a tendency to eat late at night, try eating your last meal earlier in the evening and giving yourself a longer fasting window before bedtime.
A good fasting window should be sustainable and easy to consistently stick to in your routine. A common misconception is that fasting should be hard, grueling, or downright unpleasant.
While some people may find challenging fasting schedules to work for them, it is best to start with an easily achievable fasting window and build up from there if necessary.
As with most successful lifestyle changes, consistency is a must for seeing long-term results. If you set a fasting window that is too challenging or unrealistic for your lifestyle, there may be an increased likelihood of you giving up and abandoning your IF routine altogether.
Allows for Flexibility
Finally, a good fasting window should allow for flexibility to accommodate special occasions or unexpected events. While it’s essential to maintain consistency with your IF schedule, it’s also important to be forgiving and adaptable.
If you have a social event that falls outside of your typical eating window, you shouldn’t stress about it. Enjoy the occasion and get back on track with your fasting schedule the next day. Intermittent fasting is a flexible lifestyle and it’s okay if you make occasional exceptions.
How Long Should My Fasting Window Be?
Your fasting window should be long enough for you to experience the full benefits of intermittent fasting, but not so long that it becomes unsustainable or unhealthy.
Here are some windows that are thought to be long enough to provide intermittent fasting benefits:
- 12 hours – the minimum recommended fasting window for beginners. After 12 hours, your body starts to experience significant changes in blood sugar and insulin levels.
- 14-16 hours – the most common fasting windows for intermediate fasters. After 14-16 hours, your body will enter the state of ketosis, where it starts burning stored fat for energy. Other benefits, such as improved mental clarity and reduced inflammation, may also be experienced.
- 18 – 20 hours – the most challenging but potentially most beneficial fasting windows. After 18 hours, your body is thought to increase autophagy, a natural process of cellular repair and regeneration.
The benefits of longer fasting windows can include improved immune function, increased longevity, and accelerated weight loss (11).
Below are some fasting windows where you’re more likely to experience intermittent fasting side effects and they may not be suitable for everyone:
- 23 hours aka OMAD – also known as one meal a day, this fasting schedule involves consuming all your daily calories within a one-hour window. Some people may thrive on this schedule, but it may be too restrictive or unsustainable for others.
- 24+ hours – also known as extended fasting, this involves going without food for a full day or longer. It may have significant benefits, but it’s not recommended for beginners and should only be done under the supervision of a healthcare professional.
Is a Longer Fasting Window Better?
A longer fasting window may lead to more significant results. Some intermittent fasting benefits that can be enhanced with longer fasting windows include weight loss, improved insulin sensitivity, and increased autophagy (11).
However, this doesn’t mean that a longer fast is always better. Firstly, it may not be sustainable for everyone, and pushing yourself too hard can result in burnout or unhealthy habits.
In addition, a longer fast is not necessarily more effective if your diet is unhealthy or unbalanced.
If you don’t prioritize nutritious snacks and meals within your eating window to support overall health and maximize the benefits of intermittent fasting, then the length of your fasting window may not make much difference.
You can read more on eating well in our intermittent fasting 7-day meal plan guide to get you started on the right track.
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Frequently Asked Questions
Is an 8-hour fasting window good?
An eight-hour fasting window can be a good starting point for beginners, but it may not provide all of the potential benefits of intermittent fasting. The benefits of intermittent fasting typically become more noticeable after 12 hours or more of fasting.
What is the most effective fasting schedule?
The most effective fasting schedule is the one that works best for you and your lifestyle. There is no one-size-fits-all approach to intermittent fasting, so you should experiment and find a routine that suits your individual needs and goals.
That being said, a common and popular approach is the 16/8 method, which involves fasting for 16 hours and eating within an eight-hour window. This schedule allows for significant benefits while still being flexible and sustainable enough for most people.
Why am I not losing weight on 16/8 fasting?
You’re not losing weight on 16/8 intermittent fasting because weight loss is not solely dependent on your fasting schedule. In order to lose weight, you must create a calorie deficit by consuming fewer calories than you burn through daily activities and exercise(5) (12).
If you’re not seeing weight loss results with 16/8 fasting after one month, you may need to reassess your diet and make sure you’re consuming enough nutritious food within your eating window, but not overeating.
You may also want to consider incorporating strength training or other exercises as a means of boosting metabolism and muscle growth.
Can I eat anything during the fasting window?
If you are doing time-restricted feeding, then you should typically only consume non-caloric beverages such as water, black coffee, or herbal tea during your fasting window. Consuming any food or caloric beverages will break your fast. However, some versions of intermittent fasting, including the 5:2 diet, allow for up to 500-600 calories on fasting days.
Does coffee break a fast?
Coffee can break a fast if it contains any milk, cream, or sugar. However, black coffee without any additives is generally considered to be safe to consume during a fasting window as it contains very few calories and does not trigger an insulin response.
The Bottom Line
A good fasting window for intermittent fasting should be long enough to induce ketosis, align with your body’s natural circadian rhythm, allow sufficient time for nourishment, and accommodate physical activity. It should also allow you to consistently stick to your IF routine.
Finding the right balance of these factors can help enhance your experience and maximize the benefits of intermittent fasting for your health and wellness journey. So, you should experiment with different fasting windows, listen to your body, and find what works best for you.
This article is intended for general informational purposes only and does not address individual circumstances. It is not a substitute for professional advice or help and should not be relied on to make decisions of any kind. Any action you take upon the information presented in this article is strictly at your own risk and responsibility!
- Circadian Clocks and Metabolism (2014, ncbi.nlm.nih.gov)
- Circadian rhythms, time-restricted feeding, and healthy aging (2017, ncbi.nlm.nih.gov)
- Circadian rhythms: a regulator of gastrointestinal health and dysfunction (2019, ncbi.nlm.nih.gov)
- Exercising in the Fasted State Reduced 24-Hour Energy Intake in Active Male Adults (2016, hindawi.com)
- Fat loss depends on energy deficit only, independently of the method for weight loss (2007, pubmed.ncbi.nlm.nih.gov)
- Optimal Diet Strategies for Weight Loss and Weight Loss Maintenance (2020, ncbi.nlm.nih.gov)
- Physiological responses to acute fasting: implications for intermittent fasting programs (2022, academic.oup.com)
- Physiology, Circadian Rhythm (2023, ncbi.nlm.nih.gov)
- The Effects of Intermittent Fasting Combined with Resistance Training on Lean Body Mass: A Systematic Review of Human Studies (2020, mdpi.com)
- The Health Impact of Nighttime Eating: Old and New Perspectives (2015, mdpi.com)
- Unravelling the health effects of fasting: a long road from obesity treatment to healthy life span increase and improved cognition (2020, ncbi.nlm.nih.gov)
- “Calories in, calories out” and macronutrient intake: the hope, hype, and science of calories (2017, journals.physiology.org)