I too did the phospho/fleet prep and my first words to my GI the day of my c/oscopy and e/oscopy was, "I am never taking that crap again, so you better get some good pictures of my insides!" I am not a wussy at all, have had 2 kids, no drugs, etc., but aside from food poisoning once, this was hell, absolute hell. The procedures.....what a walk in the park!
Okay, so to answer your question, my instructions from my GI said to start mid afternoon with one bottle, then early evening with the second and last bottle. Yeah right. Good thing I don't stick to instructions! I started at 9:30 in the morning, and I am soooo glad I did. I finally finished using the facilities at 8:30 that night, 11 hours of you know what. I would have hated to have been up all night long. If I had been, I would not have made it to the hospital for the procedures! The next morning I woke up so dehydrated, could barely stand to shower and could not even hold the blow dryer to dry my hair. Good thing they said "no makeup" as there was not going to be any of that anyway! So, I started much earlier than they told me to, and no, my procedures were not cancelled and I was perfectly cleaned out, and had a good night's sleep! Perhaps call your dr. and ask if you can start earlier, I don't want to mislead you, but just wanted you to know my experience last week. I was also using the facilities every 4 minutes, so how much sleep would I have gotten? None!