Here's one reason why it might be a
bad idea -
About 2 years ago, we got a call for an unconcious person in a vehicle on the side of the road. I was at the fire station when the call came in, and responded with the fire engine. This meant that, going down the road emergency and in a line were a fire truck, an ambulance, my truck, and a deputy who also happened to be at the station.
The road this was reported on was a loop through a neighborhood, and we went in the first entrance. However, the car was located on the other side of the loop at the second entrance, which means we got to do this little procession through the neighborhood, bringing the residents out of their houses out of curiousity.
So by the time we get to the car, we have a fire engine, an ambulance, my truck, a deputy, (making a total of 7 people) as well as about 10-12 neighbors. Turns out to be some teen that fell asleep in his car over night, and slept through the rising sun.
So, unless you LIKE waking up to about 20 people, be careful where you sleep if you do sleep in your car.