Used to have a girlfriend that was terrible about that. Had a Heritage then so something like the King pak with the arm rests to keep someone inboard wasn't an option. Would be going along and suddenly "boink." Passenger helmet bouncing off my head or helmet. Her finally being sent packing was at least partially resultant from not being happy about being told she just couldn't ride anymore. It is freaky knowing someone might just be flopping around back there.
That's how I figured out that she was sleeping, mostly long road rides, like the Interstate, but have to slow or brake quickly.......boink, Hey, what the hell you doin' back there (intercom) uhh, guess I fell asleep. Before or after you finished your latte? Before....but I didn't spill any

Geez, what the hell. It's considered her bike and she'll let me do anything I want to it, just like...................TMI

Thank goodness she's a leightweight, my fat arse seems to handle it and I actually never really know if she's asleep unless I get the helmet boink.