Don't worry about it too much if you aren't able to get a full night's sleep the night before the interview. Most people are fully functional on a couple of hours as long as it's just for one night. Just make sure that your alarm is turned up all the way and back-up alarms are always nice too (I always carried a travel alarm clock along with setting the hotel alarm clock). Ironically, if you focus too much on getting a full night's rest, you will never get a full night's rest. At least that's what happens to me.