Out of my own experience—a daily AutoPilot user for the last 3.5 years—when you’re tired, it is easier to fall asleep with AutoPilot than without AutoPilot. Especially in the evenings on the freeway when it’s dark.
I agree with the majority in the autonomous world that driver monitoring is going to be important for the years where we gradually transition from driver supervision to full self driving.
It's a difference of opinion here. Some people are more alert with AP on because they know they are beta testing and helping the system train. I find myself to have similar drowsiness with AP on or off. I did find myself hitting lane markers yesterday with AP off and drowsy..so it's definitely safer to have AP on when you are that tired.