As a rule, roomies should agree that sleep shouldn't keep the other from sleeping. To avoid conflicts, you and your roommate might agree not to use your dorm room as your main place to study. But one thing you're both usually going to want to do in your room is sleep.
Make sure you talk about everything you need to get a good night's sleep. Odds are, you'll have different preferences about time, temperature and noise, just for starters.
Does one of you like the room chilly, while the other wants it warm? Rather than waging a battle over the thermostat or fan, strike a compromise. Agree on a temperature. If it's chillier than you like, use a blanket or comforter to keep warm.
If one likes to sleep with music in the background but the other wants silence, earphones for the music lover or earplugs for the quiet lover might be in order. How about lights? If one must have a nightlight, put it where it won't reach the eyes of the roomie.
Time may be the most important consideration. Agree on quiet hours and stick to them. Don't set a loud alarm clock to go off in the wee hours. If there's an unusual circumstance that requires extra-early rising for one person, maybe a discreet cell-phone alarm will do the trick. And agree that the one who rises early or stays up late will make a real effort to be quiet.
Speaking of sleeping: Read on for thoughts about sleepovers.