Let's get this out of the way now -- too much alcohol does a whole host of horrible s**t to just about every organ in your body. It's just that your brain isn't really one of them. Your alcoholic uncle doesn't actually get any stupider when he drinks. Meaner, but not stupider.
"Asleep" isn't the same as "brain damaged."
The whole idea of alcohol destroying your brain actually dates back to the temperance movement in America, circa 1830. Using the now familiar tactic of "It's OK to lie as long as it's for a good cause," the writers of the time claimed that alcohol would do everything from destroying your brain to making you catch fire if you drank too much. Though the latter, if true, would make losing a few brain cells seem fairly minor.
Plenty of people believed it -- it just seems like common sense that beer destroys the brain. Have you ever seen a drunk person before? And listened to his jokes?
"So this guy, he turned and said ... and said ... s**t. It was something about priests, or bears. Hey -- QUIT LAUGHING AT ME!"