I've never dozed off on a motorcycle or scooter, but I know it's happened in a car in my younger days. I used to drive alone from Maine to southern Virginia without a stay over, stopping only for gas, coffee and pee breaks. Of course, this was 40 years ago when I was young and stupid. I would zone out for a few minutes, then jerk back awake. As I got older, I got wiser and broke that habit by pulling into a rest area and grabbing a quick nap whenever I felt my eyes closing.
A number of years back, heading home to Cleveland from a BMW rally in Trenton, Ontario, Canada, my wife was on the back off my BMW R touring bike, and she does not remember riding through Pennsylvania. She was wedged pretty securely between me and the back rest, and a couple of duffle bags bungeed to my saddle bags. She was sitting in a frigging easy chair. Anyway, that spooked her so much that she gave up long distance riding with me.