Is it illegal to sleep in the back of my car while drunk?

A mate of mine got done for it after he had lost his house keys and then went out for a drink, ( on foot) when he got home he still was waiting for his son to arrive home so he could get in. He then got into the back of his car after hiding his keys on top of a wheel. A policeman came along and after a search found the keys so he was successfully prosecuted. Makes you wonder doesnt it. IT seems unfair to me