Based on my own research, a three-day window of modified fasting is simply more doable for the vast majority of people. And that three-day window is enough time for your body to do some serious work in autophagy, clearing out damaged cells, reducing inflammation, and improving so many markers of good health.  If you want to extend your fast to five or six days, all the better! But I find that people who set out with a specific goal of five days, instead of three, have high dropout rates. Three days seems to be the magic number for a doable, realistic fast that doesn’t interfere with your social life.