Why preparing for a colonoscopy is a two-step process

I was flabbergasted to learn, as part of the preparation for a colonoscopy, I would not only have to start the bowel clean-out the night before, but would have to get up in the middle of the night to finish the job. If I hadn’t eaten for a day and had dutifully drunk that horrible stuff and suffered the consequences by bedtime, why would I have to do it again in the wee hours?