Here in the Western USA, I have slept in my car many times -- and have been woken up many times by the police. Each time I have told them that I got too tired to drive. They have always left me be after that...they don't want tired people driving (I think that is the second most common cause of wrecks next to drink).
But what I hate (and it has happen many times) is when I park and lean back in the driver's seat to catch a few winks -- I wake up thinking I have fallen asleep at the wheel. I grab the steering wheel, slam on the brakes and try to see out the window. I finally will realize that everything is dark and the car is not moving...but my heart is beating a mile a minute, so I get back on the road wide awake. Man I hate doing that...I have to lie down across the seat or something to keep it from happening (hard to do when I was traveling in a VW bug with the backseat full of stuff.)