I used to have a nice little NASA nap when I was doing single pilot night freight in Metroliners. The radio was very quiet at that time of night and the autopilot seemed reliable enough. The three hour sectors were so boring.
I did do some unplanned NASA napping one day on a passenger charter in a Baron with no autopilot. I woke up after about 40 seconds of slumber to see a 1500 fpm descent and 45 degrees left wing down. I casually looked over at the front seat passenger to notice that he was looking out his side window and had not noticed. He smiled at me and seemed to be quite happy with the way things had been progressing.
That really made me think about a subtle incapacitation scenario (eg falling asleep) and how far the pax would let it get out of shape before they would feel the necessity to have a go themselves. Talking single pilot here of course.
As for two crew ops, every company I have ever worked for has an ops manual requirement that First Officers are prohibited from sleeping whilst in flight.
Sounds very reasonable to me!