I always used to do things like that too. I don't know why but it was so soothing. Now I have naps in the car. I think it's the mix of the lovely hau5 music playing through my iPod, the bumps the car goes over, and the slight rumble of the engine that makes me sleep so deeply. It all creates a kind of rocking sensation. The best sleeps I have are those in the passengers seat of a car.