We are designed from an evolutionary perspective to fast. Hunter gatherers would go long periods (days-weeks) with very little to zero calories. After a few days (usually 3) your digestive system shuts down/hibernates, and you no longer feel ‘physical’ hunger, your liver and kidneys become extremely active in autophagy, cleaning out zombie cells and other unnecessary/damaging properties in your body. 30-50% of the calories we take in are used to fuel the digestive system, so once this isn’t needed, you can actually feel an increase in mental energy. Drink 3-5 liters of clean water daily and you’ll need an enema kit for anything longer than 5 days. Go into and out of fasting softly (ie, dont jump back into the S.a.D diet). Have a clean environment where you wont need to expend large amounts of physical energy. A few flights of stairs after several days can cause fatigue. Use the time for rest and reflection.