Substances use and abuse. Alcohol.
Alcohol causes most people to feel sleepy. There is a paradoxical reaction in some people, and it causes them to be more rowdy. Even in those who fall asleep more easily, the sleep quality is not as good for a variety of reasons.
The withdrawal from the chemical effects often occur in the middle of the night. Withdrawal effects are the opposite of the initial effects. So that means your blood pressure increases, heart rate increases, and you're tossing and turning more, whether or not you realize it.
Alcohol is a diuretic, so you end up going to the bathroom in the middle of the night/early morning. If you're living in a dorm, you may have to climb out of the loft bed, find your room keys and slippers, sneak out of the room without disturbing roommates, walk down the hall to the bathroom, turn on the bright fluorescent lights in there, use the bathroom, come back, and climb back up into the loft bed as quietly as possible in dim light. Going to the bathroom is a big production that severely interrupts sleep!
To break down alcohol, the liver has to change the chemical several times before it becomes harmless. Some of the intermediary stages make people feel poorly. If there are enough intermediary stages hanging around the next morning from binge drinking, then you get the sensation of a hangover.
Some people drink to help them relax. That's a dangerous justification for the use of alcohol. It's basically self-medicating with a very addictive drug. It is very likely that tolerance will build, and you'll need to use more and more alcohol to achieve the same effects. If anxiety is preventing sleep, talk to either a doctor or a counselor about the problem.
(I must admit that I didn't take my own advice just now. I decided to invent SleepPhones instead so that I could just listen to a hypnosis track whenever I had too many thoughts keeping me awake. Using headphones to listen to someone talking me into relaxing was a great natural cure for my insomnia.)
It is common knowledge that alcohol help people relax. But hopefully I've made it clear why alcohol use only reduces sleep quality and can potentially be problematic. Since most drinking in college is underage drinking (illegal), it's by definition, alcohol abuse.