When I sleep I slump over almost into a fetal position. It's never comfortable for me to sleep in a car. My Mrs on the other hand appears to be awake and sitting up, the only subtle clue is a slight bobbing of the head if her sunglasses are on. I wonder how engagement detection would work in her case? There have been many times I have been having a grand old chat with her as I am driving and in my periphery and she is sound asleep. I predict that she could easily fool the system unto thinking she is awake.