That's the way that toxins leaving the body make a person feel too. When juice fasting it is important to get the toxins out quickly, before they are reasorbed back in to the system by doing pure water enemas, and coffee enemas, or salt water flushes, or activated charcoal.
When I fast I drink a lot of water, More than 8 glasses a day, along with drinking herb teas, lemon juice, 'Recarge', Energen-C, vegtable broth, and 2 -8 oz. glasses of apple juice and at least 3 8 oz, of vegetable juice. I drink something a least every hour.
The appetite and urge to eat leaves after the first day or two. I usually don't do much during a fast. Read, meditate, little or no tv, juice, enemas, a little walking, nap, soothing music, yoga, the minimum - sometimes massage - I pamper myself.