I recently had a colonoscopy. People react differently to sedation. I've had people tell me they slept through the entire thing. I have an extremely high tolerance for medication, so I just got a mild buzz. They pumped something into me whenever I cramped though, to kill the pain.
Serge, tell them you have a very, VERY low pain threshold. Turn into a whiney weakling if you have to. Make sure they give you enough juice.
Other than that, just relax and watch the monitor. It's really fascinating. And keep repeating to yourself "doing this could save my life, doing this could save my life." That really helps a *lot.*
My Dad had colon cancer, they caught it early, and other than having to have a lot of colonoscopies, that's all that came of it.