How to Choose Your Ideal Fasting Window
Intermittent fasting (IF) is not like other diets—it’s a lifestyle with known health benefits. Just like other lifestyle choices, there is room for flexibility and variability to meet your personal needs. Fasting does not have to fit a “one-size-fits-all” kind of approach – choose what makes you feel best and is sustainable.
If you are new to fasting, here is what to consider when choosing your ideal fasting window and why.
Figure Out Your Fasting Goals
When you make the decision to start fasting, the first things you should consider are your purpose and goals for fasting. Are you looking for longevity? Trying to lose weight? Or are you trying to gain clarity and focus during your day?
Setting goals for health behaviors is key to making sustaining improvements to your health. Research supports making SMART goals (Specific, Measurable, Attainable, Relevant, and Time-based) to turn your intentions into actions. [1,2] SMART goals increase motivation and help clear up confusion around what specific actions you want to take for your health – in this case, the action of starting a fast.
Depending on your goals, you may want to choose a different style of fasting to maximize your results.
Studies find that Time-Restricted Eating (TRE) may be effective for supporting longevity due to its interactions with our body’s circadian rhythms. Fasting windows of at least 12 hours can strengthen our body’s circadian clock, which allows for certain regulatory processes to repair cells and tissues, promote proper fluctuations in hormones, improve sleep, and potentially reduce risk of chronic disease. 
In regards to weight loss, several styles of intermittent fasting (IF) have been found to be equally as effective as calorie-restricted diets in helping individuals lose weight. Alternate Day Fasting (ADF), a method in which you eat less than 500 calories on fast days and then eat normally during a 12 hour window on feast days, is a popular and effective version of IF that has shown promise for weight loss. 
This intermittent fasting method may also play a role in protecting against chronic diseases such as type 2 diabetes and cardiovascular disease. Smaller studies have shown that ADF can increase insulin sensitivity, decrease serum cholesterol and triglyceride concentrations, reduce blood pressure, and decrease inflammatory markers. The 5:2 method (where you eat 5 days out of the week and fast for two non-consecutive days) has been shown in small, short-term studies of overweight individuals to be effective in decreasing insulin resistance in addition to reducing body fat, though long-term studies are needed to show if these benefits are sustained. 
Start off Slow
There is no need to rush into an intense schedule in order to get a start on your fasting journey. It may in fact be easier to adjust to fasting and maintain it long-term by starting off at an easy fasting goal and working your way to your ideal fasting window.
Time-Restricted Eating (TRE) is a great way for beginners to IF to help their bodies adjust to a new eating style. To begin, you can try a 12:12 restricted eating pattern, where you fast for 12 hours and eat during a 12-hour period.
If this is similar to how you normally eat, then starting with a 14:10 fasting window may be better to give your body a challenge to adjust to fasting. Allow yourself between two to four weeks for your body to adjust to one fasting window before moving onto a more restrictive window. For TRE, many people end up with a 16:8 schedule and a few individuals choose a 20:4 schedule.
When your body becomes used to TRE, trying fasting schedules like the 5:2 method, alternate day fasting, or the eat-stop-eat method (where you schedule 24hr fasts 1-2x/week) may become somewhat easier for you to follow. Please note that if you are considering a prolonged fast of 24 hours or longer, it is important to discuss with your healthcare professional prior to starting. In addition to these fasting methods listed here, there are many different types of fasts that you can explore to find your ideal fasting window.
Once you’ve started trying a fasting method, take care to have a nutrient dense diet and increase your water intake to meet your needs and reduce side effects of fasting.
Listen to Your Body
You are the ultimate expert in knowing how your body feels. It is important to monitor how your mind and body feels while adjusting to a fasting window.
As mentioned earlier, it does take some time for your body to adjust to an IF schedule. Some people may experience slight headaches, feelings of hunger, lower energy, and food cravings when beginning a fast. These symptoms may resolve after two weeks of following a particular fasting schedule.
However, there are symptoms that indicate that a fasting window is too long for you to realistically sustain. These symptoms include severe headaches and dizziness, nausea and vomiting, significant GI distress, extreme weakness and loss of consciousness, hair loss, amenorrhea, negative mood changes, and ongoing food obsessions.  If you are experiencing these symptoms several weeks after starting your fasting journey, it is best to stop your current fast and speak to a health professional about a sustainable fasting schedule or if fasting is right for you at all.
If following the appropriate fasting schedule for your body, you may feel slight reductions in appetite that help you manage your intake, increased focus and productivity, and improvements in body composition.  There is limited research to date to support long-term outcomes, however many fasting experts who embrace fasting as a lifestyle strategy believe that a variety of health benefits may continue especially if you are able to consistently follow your fasting window long-term. Further research on the impact of fasting on chronic disease prevention, reduced inflammation, and longevity. As research becomes available, this will make fasting even more attractive to even more people.
Once you are settled into your ideal fasting window, you can adjust the times in which you eat to better suit your daily activities. Your start and stop window may vary if you are a night owl or early bird, engage in certain types of physical activity, or if changes in your regular schedule occur.
For individuals with existing chronic conditions, it is recommended that you consult a health professional such as a doctor or registered dietitian nutritionist prior to starting any type of fasting. Women who are pregnant or breastfeeding, elderly persons (especially those at risk of undernutrition), people with a history of disordered eating, those with cancer or insulin-dependent diabetes, or anyone under 18 should not attempt fasting.
By Markita Lewis, MS, RD
Markita has an interest in the biological, social, and cultural aspects of eating. She enjoys writing about nutrition and wellness, food justice and policy, cultural foodways, and the psychology of nutrition. You can find her at: www.wellnessandchill.com