Intermittent fasting is known for being a customizable eating routine that can lead to weight loss, fat loss, and tons of other potential health benefits. Under the general umbrella of intermittent fasting—an eating routine that focuses solely on when you eat rather than what you eat—there are many different types of intermittent fasting schedules. These different schedules might be more or less effective for certain health goals—and equally they may be more or less suitable for different individuals.
So, where does 12-hour intermittent fasting land in that spectrum? How does 12:12 intermittent fasting work, how does it compare to different fasting methods, and is fasting for 12 hours enough to achieve your goals?
Let’s dive in.
Whether you’re considering trying intermittent fasting for the first time or you’re looking to switch up your schedule, you should always discuss any changes to your lifestyle or eating pattern with your primary care provider before you make them. You should be particularly cautious about any form of intermittent fasting if you:
- are pregnant, breastfeeding, or trying to conceive;
- are under 18 or over 80 years;
- have a BMI in the “underweight” category;
- have a history of, or are at risk for, disordered eating;
- are under a lot of stress;
- have nutritional deficiencies;
- have an existing health condition (like diabetes or insulin sensitivity); or
- take medications that affect blood sugar or blood pressure or need to be taken with food.
What Is 12-Hour Intermittent Fasting?
A form of time-restricted eating, 12-hour intermittent fasting involves dividing your days into a 12-hour eating window and a 12-hour fasting window. That means that for 12 hours of every day, you take a break from consuming calories. For example, if you finish dinner at 8 p.m., you could fast until 8 a.m. the next morning.
When that fasting window occurs is completely up to you. If you’re an early riser and like breakfast first thing at 5 a.m., you could aim to finish eating by 5 p.m. the previous day. If your schedule means you’re not finishing dinner until 9 p.m., no problem—just hold off on breakfast until after 9 a.m. If you work the night shift, maybe your eating window actually runs from 3 p.m. to 3 a.m.
Whatever hours you choose, the beauty of 12-hour intermittent fasting is twofold:
- Compared to other intermittent fasting schedules like 16:8 or alternate day fasting, this intermittent fasting schedule involves a decently long eating window and a fasting period that’s mostly taken up by sleeping.
- Beyond the number of hours you fast, there are no hard-and-fast rules for this eating plan. You can eat and drink whatever you want—as long as it’s during your eating window. (Yes, there are certain choices that are more health-promoting and nutrient-dense than others, but intermittent fasting is built around when rather than what you eat.)
What Happens After 12 Hours Without Eating?
After you finish eating, your body gets to work digesting, distributing, and absorbing the nutrients you just consumed. Insulin—a hormone that ferries nutrients to where they need to go—takes center stage in that process.
Over the next few hours, what you ate gets transformed into useful body stuff, like fat, muscle, energy, and brain power. Once everything has reached its destination, your body enters a post-absorptive state—basically the internal equivalent of a post-prandial snooze—for roughly 8–12 hours before your insulin levels drop and you enter a fasted state where your body is primed for fat burning.
So, factoring in the first couple hours post-meal that your body needs to transform nutrients, is a 12-hour fast enough to get you to that fasted state? Maybe, maybe not. Bodies go through this process at different speeds, and there’s not an exact science for determining how long your body needs to reach that fasted state.
But is a 12-hour fast enough to get the benefits of intermittent fasting? Let’s take a closer look.
12-Hour Intermittent Fasting and Weight Loss
If a 12-hour fast is enough for your body to reach that fasted state—otherwise known as metabolic switching or entering ketosis—you will start burning fat for energy rather than the glucose (sugar) you get from meals. Fat loss and weight loss aren’t the same thing, but since weight loss involves a general decrease in total body mass (including fat, muscle, and water), burning fat leads to losing weight.
Regardless of whether or not your body reaches that fasted state, though, a 12-hour intermittent fasting diet can be an effective way to lose weight for two reasons:
- It creates a calorie deficit. If you’re eating for less time than you normally would, you’ll likely lower your calorie intake, too.
- A more limited eating window could lead to making more health-promoting and nutrient-dense food choices. Having a limited timeframe for your daily meals might mean you’re more focused on getting all the nutrients you need (and thus have a more balanced diet) while leaving less room for snacking and late-night treats.
While your weight loss success with 12-hour intermittent fasting will depend on a lot of factors—like your general lifestyle, your eating pattern before intermittent fasting, and what you consume during your eating window—a 12-hour fast does still have potential weight loss benefits.
12-Hour Intermittent Fasting and Fat Burning
Whether a 12-hour fast supports fat burning depends on whether 12 hours is enough for your body to enter that fasted state. If the metabolic switch from using glucose to using fat for energy happens within those 12 hours, you’ll burn fat with a 12-hour fast.
However, extending the fast (to 14 or 16 hours) could deplete your glucose stores more, which in turn could lead to greater fat burning.
Potential Health Benefits of 12-Hour Intermittent Fasting
Unfortunately, there isn’t much scientific research that focuses specifically on the effects of 12-hour intermittent fasting, but beyond weight loss (or weight management) and fat loss, a 12-hour intermittent fast can also introduce other potential benefits associated with intermittent fasting in general. Let’s run through some of the overall benefits of intermittent fasting.
Improved Metabolic Health
Intermittent fasting can reduce insulin resistance and blood pressure while lowering cholesterol and blood sugar—all of which support better metabolic health and reduce risk of metabolic diseases like Type 2 diabetes.
Improved Heart Health
By reducing cholesterol, blood sugar, and insulin resistance, intermittent fasting may also support better cardiovascular health and the prevention of cardiovascular disease.
Improved Gut Health
Intermittent fasting can increase the richness and diversity of gut microbiota (bacteria and other microorganisms that support digestion, nutrient absorption, and immune system functions), which in turn can boost gut health and metabolic health markers.
Improved Brain Health
Particularly for older adults, intermittent fasting could improve brain function and reduce cognitive impairment.
Reduced Inflammation
Inflammation has negative impacts on nearly every bodily system—like cardiovascular, neurological, metabolic, autoimmune, and respiratory, just to name a few—and intermittent fasting can improve inflammatory markers.
Potential Health Risks of 12-Hour Intermittent Fasting
With a 12-hour fast, potential side effects and health risks are more limited, as it simply extends your body’s natural fasting period—when you’re asleep—by a few hours.
That said, there are always safety concerns to be aware of, even if your primary care provider has cleared you for fasting. Typically any negative health impacts from a 12-hour fast will be mild and fleeting—and usually disappear all together once you adjust to your new eating routine—but if any are persistent or severe, it’s a sign to stop fasting and talk with your doctor to ensure you don’t put yourself at risk for more serious health problems like malnutrition or disordered eating.
Headaches and Lightheadedness
Fasting can leave you more prone to dehydration as well as caffeine withdrawal and low blood sugar, which can contribute to headaches or wooziness.
Fatigue and Low Energy
Until your body adapts to this eating routine, you may experience a drop in energy levels as it adjusts to eating less and less frequently. Likewise, low blood sugar can also lead to brain fog.
Irritability and Other Mood Changes
When you’re new to intermittent fasting, some studies suggest this type of eating routine could increase negative emotions. Equally, until you get used to a different eating schedule, hanger (and the resulting stress and crankiness it can cause) is pretty common.
Poor Sleep
Existing research has mixed opinions on the effects of intermittent fasting on sleep, but some suggest it can affect sleep quality and sleep duration.
Digestive Issues
Changing your eating habits means the organisms in your gut—which help regulate your digestion—have to adjust to a new schedule. In the interim, you might experience bloating, constipation, diarrhea, or nausea. Dehydration can also lead to feeling more blocked up.
Pros and Cons of 12-Hour Intermittent Fasting
Factoring in all the logistics and possible benefits or risks, what are the pros and cons of the 12-hour intermittent fasting diet?
Pros
12-hour intermittent fasting can lead to weight loss as well as a slew of other potential health benefits associated with intermittent fasting. It doesn’t require a major overhaul of your eating routine, and nor does it have prescriptive rules about what you can eat, so it’s fairly flexible and easy to maintain compared to some other diet plans. You can tailor both the timing of your fasting window and what you eat during your eating window to what works for you. A shorter fast also means any side effects are more likely to be minimal and short-lived.
Cons
As with any fasting schedule, there are still potential risks involved, and it may not be a safe or effective solution for everyone. The shorter fasting window may be easier to manage, but it may not be as effective for certain health goals, like fat burning, when compared to other fasting methods. There’s also not much research specifically on 12-hour intermittent fasting, so what we know about the benefits and risks are more affiliated with intermittent fasting in general and not this specific eating routine.
Is 12-Hour Intermittent Fasting Right for You?
So, is 12-hour intermittent fasting worth it? And if so, is it worth it for you?
What “worth it” means might depend on your goals for fasting. If you’re specifically fasting to burn fat, it may not be the best fasting schedule since 12 hours might not be enough time for your body to enter ketosis. However, if you’re looking to lose weight or to see general improvements in your health, particularly around metabolic function and inflammation, without having to change your lifestyle and eating habits too much, it could be a solid choice.
Whether this particular time-restricted eating method is “worth it” or not can also depend on where you are in your fasting journey. If you’re just dipping your toes into intermittent fasting, a 12-hour fast is a great way to test the waters. Think of it like a gateway fast: It’s a perfect place to give intermittent fasting a try without having to cope with too much change all at once. Rather than an ice plunge, it’s more of a slow ease-in to a refreshing pool. Then, if that feels comfortable and like it’s working for you, you can discuss with your primary care provider whether it’s worth transitioning into a longer time-restricted feeding schedule (like 16:8 intermittent fasting) or another type of intermittent fasting (like alternate day fasting).
TL;DR
A form of time-restricted eating, 12-hour intermittent fasting involves eating your meals within a 12-hour window and then fasting for the remaining 12 hours of the day.
As with any intermittent fasting routine, it can be a great way to lose weight, but since the shorter fast means your body may not have enough time to make the metabolic switch between burning glucose and fat for fuel, it may not be the most effective fasting schedule for fat loss.
That said, it’s still a fasting schedule that can promote a range of other health benefits—like improved metabolic, heart, brain, and gut health—and typically any side effects or health risks are minimal. Plus, since the fasting window is only a few hours more than the time you’re normally fasting every day when you’re sleeping, a 12-hour intermittent fast is an especially flexible and manageable approach to intermittent fasting.
So, if you’re trying out a fasting-based schedule for the first time, 12-hour intermittent fasting is an ideal gateway to see if this type of eating routine might be a good fit for you, your body, and your goals.
