More than once...
There is a bridge over the Saluda River on Hwy 39 and SC 34, near Greenwood State Park. I was driving my folks' '84 Scottsdale pickup back from Clemson one weekend after finals and was bushed. Back then that was a trestle bridge. I went to sleep driving over the bridge, and the truck drifted ever so gently into the guard rail. The high-pitched grinding noise woke me up and I drifted back to center lane. When I got home, I saw the rail had ground away two quarter-sized half-moon chunks out of the chrome-trim-lined fender edges. Gave me goose bumps; if that had happened 3 or 4 seconds sooner or later, I'd have been in the river. And that's a pretty good drop there.
Once when I was working on my MS, I was taking a couple of advanced EM and math classes, running an electronics lab, and doing my thesis research. And there was a girlfriend too. I was waiting in the EE building hallway on someone, and I leaned against the wall. My friend finally came along and woke me up...it's a wonder I stayed upright.
In an integral equations class I had, there were only 5 of us. Class met 5-6:15 on Tuesdays/Thursdays. The prof (also my MS advisor) used an overhead projector, so curtains were always pulled and the lights dim. OF COURSE I dozed off one day. He used a dowel rod as a pointer. He came over and SLAMMED that rod on my desk right by my head so hard he broke it. My head shot up, and he said, "I'm not mad you went to sleep, but this is IMPORTANT, and I want you to understand it!" (He was right. I still use that material to this day.) I had him for two more classes, and I NEVER went to sleep in that one again, or either of the other two. I think that's when I started drinking coffee.