No, I usually eat only two main meals on the day of the flush, I delay the lunch and snack lightly on something between meals if I feel hungry. Then do the classic H. Clark flush, just 2 hours delayed. I have followed this schedule from my very first flush and sometimes delay it even more, if needed. You just listen to your body and adjust the mealtimes accordingly. Eat 3 or 4 times if hungry, just quit in time. I believe it's important to feel comfortable. I advise you to keep drinking water as I got really dehydrated the first flush when I followed instructions and did not drink any. Just keep about 30 minutes between water and oil or Epsoms, as those need to be taken on an empty stomach.
The flush itself was originally timed at 10 p.m. because liver is most active from 10 p.m. to 2 a.m. It is recommended to sleep during this time to aid the liver in detoxing. I have however not been able to convince my body to fall asleep this early, especially during spring and summer and I see no point in just going to bed and feeling possibly nauseous for a few hours. If I drink the oil/juice concoction at my usual bedtime I have no problem in falling asleep within minutes. Experienced flushers do even daytime flushes so indeed, the schedule is not set in stone.