Ruins your restful sleep because your body does this thing where it tries to overcompensate for the depressing effect of the alcohol by overstimulating things. You may fall asleep more easily, but then your actual sleep will suck, compared to a non-alcoholic induced sleep.

Source: I read a book on it once about a year or so ago. The name of the book was [Buzz: The Science and Lore of Alcohol and Caffeine](http://www.amazon.com/Buzz-Science-Lore-Alcohol-Caffeine/dp/...).