It can take 2-4 weeks for the body to become accustomed to IF. It's best to pick your window and slowly work your way up to a 16-hour fast. For example, start with a 12-hour fast where you eat from 8am-8pm. The next time, try waiting until 10am and eating from 10am-8pm. Once you can easily do this, move to 15 hours and then 16 hours.
Once you break your fast, it’s recommended that you eat every 2 hours. Try not to undereat during your feeding time as it can slow down your weight loss. And don’t just binge or eat junk during your feeding time. Eat something healthy with protein at each interval to keep you full and support muscle mass.
Pay attention to the clock and stick to your fasting window. It may take you time to find your perfect window and that window may change over time or even during a week for a special occasion. For example, you may want to eat breakfast one morning, or stay out late with friends one evening. Plan ahead if you know this is going to happen and try to give yourself at least a 14-hour fast by stopping early the night before.
Most importantly, be patient with yourself and don’t give up! Remember, your body must get used to running on ketones during your fasting period. Some people adapt quickly, but for others it takes a while. If you find yourself getting lightheaded or fatigued at the beginning, you are going too fast. Take your time reaching your 16-hour window and let your body adapt.