In a tent on a near concrete car park at the Le Mans race track with the sound of the cars racing in the background.
On the floor of Luton airport on the way home from the Le Mans trip. It was a bus trip and I don't sleep well (if at all) on a bus and we got to the airport about 12 hours before our flight home. I also read the whole of The Hobbit during this wait.
I used to regularly fall asleep while walking home from work in my late teens (I was often out late the night before a shift). I did the same route every day and often just closed my eyes and when I opened them again I would be much further along the road with absolutely no memory of walking any of it. The scariest time was when I had made it along two streets and crossed a road while doing this.