Hey there Justme,
That has been happening to me as well. For the first time in my life, I fell asleep while driving recently. The car was set to cruise control at 120K (I'm Canadian but I think that is about 80 mph?). Anyway, I wasn't even tired but then suddenly was asleep on the highway. The rumble strip on the side of the highway woke me up seconds before I went off the road. It was SO sudden and so unlike me that I am trying to be very aware of it now. I have found myself falling asleep at the strangest times without warning so I know what you're talking about. I'm not sure what to attribute it to but I keep a bottle of "Instant Energy" in my car now as I commute long distances on a regular basis.
I am withdrawing from years of codiene abuse- 300 mg per day and am a mother of two boys.