Intermittent fasting benefits by the hour

Written by Aly Owen

Medically reviewed by Louise Bula, RD

Intermittent fasting involves a repeated cycle of eating windows (where you consume the calories and nutrients you need for the day) and fasting windows (where you take a break from consuming calories). 

There are many different ways to fast — from time-restricted approaches like intermittent fasting 16:8 to off / on approaches like the 5:2 diet, alternate day fasting, and eat stop eat — but how does an intermittent fasting timeline actually play out in your body? What sort of fasting benefits timeline can you expect — is there a fasting chart by the hour guidebook out there?

From fed to fasted, let’s explore the intermittent fasting benefits by the hour and what fasting actually looks like, inside out — plus how to supercharge eating windows.   

Need Intermittent Fasting 101 first? Check out our intermittent fasting for beginners guide. 

Key takeaways

• Fasting affects everyone differently, so you can’t predict exactly how fasting hour-by-hour will land for you. 
• Both the side effects and health benefits of fasting by hour vary from person to person, but you may start seeing some side effects a few hours into a fast and some positive metabolic shifts 12 hours in. 
• What you eat during your eating windows can affect what happens to your body during a fast. 
• Disclaimer alert: not everyone should fast (we’ll get into specifics later). 

Benefits of intermittent fasting by hour 

We know little is more low-key annoying than getting to the end of a really good proposition and discovering there’s dealbreaker fine print. 

So, let’s get some disclaimers out of the way. 

When it comes to fasting, everyone responds differently. So, while the hourly benefits of fasting follow a general timescale, these numbers aren’t exact or universal. There’s no fasting by the hour roadmap that applies to every body and every fast in exactly the same way. 

Think of it like Google Maps: sure, it gives you a rough idea of timing from A to B, but maybe you’re an extra fast walker, or there’s unexpected traffic ahead.

Similarly, “more” isn’t necessarily “better” — a longer fast doesn’t automatically translate to better results. It’s about finding your optimal fasting period where you unlock benefits without sacrificing your well-being. 

The winding route versus the straight shot still gets you to the same place eventually, and one involves more fuel and planning …

Keeping those caveats in mind, here’s roughly what to expect in terms of fasting benefits by the hour. 

0 to 2 hours

The first few hours post eating are focused on turning food into helpful body stuff, like energy, fat, muscle, and brain power. This process is powered by insulin,[1] a hormone that transports sugar from your blood to your cells, so insulin levels increase (though how much depends on your meal and your insulin sensitivity levels [2]). Simultaneously, the levels of a hunger-stimulating hormone, ghrelin [3], decrease while those of an appetite-suppressing hormone, leptin [4], increase. 

3 to 5 hours

After your body spends some hours digesting and distributing everything, it chills in a post-absorptive state (basically digestive sleepy time). Your insulin levels drop, and your body starts converting the extra sugar it stored from the meal into energy. Levels of human growth hormone (which plays a role in regulating metabolic functions, like burning fat [5]) may start to increase.[6]

5 to 12 hours

Once your body runs out of sugar, your insulin levels drop, and your body starts breaking down fat cells to use as fuel. This process, lipolysis [7], is a crucial part of reaching ketosis (a metabolic state where your body starts burning fat as energy, which is helpful for things like weight loss). As a result, your body will start to produce ketones, which are used for fuel. 

12 to 15 hours

Now things really start to heat up. This time period is likely when you’ll hit your ketone-producing peak rate, which means you’re now most likely in ketosis. 

15 to 24 hours

By now, your body is likely using fat as its main fuel source, with ketones as a close second. That means you’re getting into the full swing of fat-burning mode. Most research on time-restricted eating uses fasts up to 16 or 18 hours and suggests fasting for this amount of time is enough to reap the weight loss and metabolic benefits. While some studies focused on longer fasting windows have demonstrated promising results, too [8], we don’t think they’re necessary, and we don’t recommend fasts beyond 18 hours unless you have medical sign-off. Although rare in people without diabetes, an 18+ hour fast can be dangerous and lead to life-threatening conditions like ketoacidosis.[9] Longer fasts may also make it harder to get in all the nutrients our body needs and loves. 

24 to 36 hours

Here is when you may also start experiencing autophagy — when your body starts flushing old / damaged cellular material and regenerating healthier, fresher material. While autophagy is broadly accepted as a healthy aging hallmark, there are still a lot of questions about how the process works [10], how it helps [11], and how we can use it.[12] Plus, everyone’s metabolism and fasting responses are different, so you can’t consult a fasting hours chart and know exactly when you’ll ascend into your autophagy era. 

It’s also important to bear in mind that it’s not possible to study autophagy properly in humans (existing data comes from animal studies), so we still have a ways to go before we can prove these benefits translate to humans. That’s why we don’t recommend such long fasts, even if you’re used to long fasting days, at least without medical sign-off, for anti-aging benefits just yet. But we’re excited to see where the research takes us.

Possible side effects by hour 

Just as your fasting benefits experience is one-of-a-kind, so too is your journey with intermittent fasting side effects. Maybe your fasting by the hour chart has one or two in store for you; maybe it has a basketful. Maybe they’re short-lived; maybe they’re persistent. 

Regardless, you likely won’t hit major bumps during the first couple hours of a fast since you’re used to a few hours without food. Beyond that, some early-stage side effects may include: 

• hunger / cravings
• headaches
• lightheadedness
• fatigue
• irritability 
• poor sleep
• low blood sugar
• digestive issues

These side effects may get more severe the longer you fast and may evolve into more serious or long-term side effects (like malnutrition or hormonal changes) if left unchecked. If any of these side effects persist, make sure to see your healthcare provider, and if any of them are severe or unbearable, you should also stop fasting immediately. 

Other common benefits of intermittent fasting 

Rome wasn’t built in a day, nor are safe, sustainable results. Beyond the fasting effects by hour you may experience, longer-term intermittent fasting benefits may include:  

• reducing inflammation and inflammatory diseases [13]
• losing weight and maintaining weight [14,15]
• supporting heart health [16]
• promoting healthy brain aging [17]
• lowering systolic blood pressure [18]
• increasing metabolic rate (ensuring intermittent fasting doesn’t slow metabolism) [19]
• lowering the risk of type 2 diabetes (by reducing insulin resistance, improving insulin sensitivity, and decreasing cholesterol and blood glucose levels) [20,21]

What your fasting benefits chart features is up to your body, and fasting isn’t effective or right for everyone. We recommend getting your fasting plan signed off by your healthcare provider before changing your eating pattern, especially if you: 

• are pregnant, breastfeeding, or trying to conceive 
• have a history of or are currently experiencing disordered eating
• have a medical condition, especially those that impact the pancreas, liver, thyroid, or gallbladder
• have a BMI less than 18.5 
• are under the age of 18, or 80 years old or more
• are prescribed medication

If you’ve been given the green light, we’re here to rev you up, keep you going, and get you across your fasting finish line. Take our SIMPLE quiz to learn what fasting schedule might be a good fit for you and your end game.  

Fasting hour-by-hour meal plan 

It’s true: what to eat during intermittent fasting is entirely up to you. You can have your cake and eat it, too (literally!). 

That said, fasting and then eating bottomless buckets of wings chased with a plateful of brownies sadly won’t give you the best results (especially if you’re aiming for weight loss).

To power up your fasting timeline benefits, choose health-promoting foods that provide fiber, protein, slow-releasing carbs, and essential nutrients to curb hunger, keep your digestive system happy, and maintain stable blood sugar. Build your eating windows (and calorie intake) around fruits, veggies, whole grains, lean protein, and healthy fats like:  

• eggs (a particularly great choice for your first meal!) [22]
• turkey chili over brown rice
• bean and lentil curry
• grilled salmon with roasted broccoli
• oats with berries and a spoonful of nut butter

Staying hydrated throughout your fast is also super important. (Here are our tips on what you can drink while fasting.) 

Especially if you’re fasting to manage weight, we also recommend limiting foods that are either ultra-processed or high in trans / saturated fat and refined carbohydrates, as they can incite cravings, spike your blood sugar, and max out your caloric intake with limited nutrients.

To separate fasting foes and frenemies, you can also plan your eating windows with our how to meal plan advice.

SIMPLE’s expert opinion and final thoughts 

Unfortunately, there’s no guaranteed, universal “benefits of fasting by the hour” chart. How your body responds to fasting — and how long it takes to reach certain milestones — isn’t an exact science. 

Everyone has a different optimal fasting schedule, so it’s about finding what works for you while keeping some benchmarks in mind if you’re aiming for certain goals. How you approach your eating windows can make a difference, too.  

We may not be able to predict your exact fasting hour-by-hour chart, but we can help you map your ideal route to success — whatever that means for you. By taking our SIMPLE quiz, you’ll get a personalized plan and expert navigation to help you steer toward your destination.

Frequently asked questions 

What hours are intermittent fasting most effective? 

What hours are intermittent fasting most effective depends on how your body reacts to fasting and what goals you have for fasting (or how you define “effective”). If you’re fasting to reach ketosis — where you start burning body fat over sugar — to aid weight loss, you’ll need to fast for around 12 to 16 hours. 

Is a 12-hour fast better than 16 hours?

Every body responds to fasting differently, so there isn’t a universal ranking system of which fasting schedules are better than others. To figure out “is a 12-hour fast better than 16 hours” for you, factor in your needs, goals, preferences, and lifestyle. (You can also take our SIMPLE quiz!)

Is 14 hours enough for intermittent fasting?

Whether 14 hours is enough for intermittent fasting depends on why you’re fasting in the first place and how your body responds to the process. Not sure your intermittent fasting hours chart is lining up? Check out our “intermittent fasting not working” troubleshooting guide. 

Is it okay to fast 72 hours every week? 

We don’t recommend 18+ hour fasts without medical supervision, so we don’t think it’s okay to fast 72 hours every week if you’re talking all at once. If you mean a weekly total (~10 fasting hours each day), a daily fast is okay if it’s safe for you in general — and you’re still getting the nutrition you need in your eating windows and not compromising on health / well-being. 

What diseases does fasting cure?

Asking, “What diseases does fasting cure?” is kinda like asking, “Why does chocolate make people happy?” — it’s getting a little ahead of the game and assuming causation and universality. Fasting may be helpful in managing and treating type 2 diabetes, preventing aging-related diseases, and improving cardiometabolic risk factors, but it’s not a guaranteed, universal outcome.[16,20,23]

Written by Aly Owen

Medically reviewed by Louise Bula, RD