From what I understand being drunk when going to sleep relaxes the muscles around your throat increasing your chance of apnea. The relaxed muscles around the throat means that the airway closes yielding a similar effect to choking (which is essentially what apnea is). The end result is that you choke periodically throughout the night meaning that the quality of your sleep is much inferior to what it could be; this is probably a large contributing factor to hangovers.

Edit: My source is a knowledgeable person on the subject, feel free to contradict or doubt the truthfulness of the answer - makes sense to me at least.

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/...).

I was shocked by this omission too, just for the shear fact it's a depressant that's usually taken in high doses right before the sleep cycle. I'd be willing to bet, especially in college Alcohol has more effect than caffeine on you sleep schedule.