I slept in my car once, some years ago. I had been out with friends, got back to my parents' house at a very late hour when they were undoubtedly in bed, then realized I didn't have my key on me. I didn't want to wake them by knocking on the door or ringing the doorbell, so I got as much sleep as I could (About 3-4 hours) in the backseat of my car. Definitely not the most comfortable experience, but again, I didn't want to wake my parents. Then, of course, the next morning when I got in and they learned what happened, they said I should have knocked or rung the bell, they wouldn't have minded getting up to let me in.