Bibi, this is a really good question and understandable concern. I think majority of people deal with an issue once in a time with a roommate.
This is bound to occur with at least one of you. It can be difficult, depending on the rooming situations, to be in close quarters with someone spending every waking moment together. You have to respect their preferences, and at times compromise. I think it would be good to ask questions about space, boundaries, and habits during the first few days together, then you can know how to respect their boundaries, and they can understand your point of view. I would make a list of things that are important to you. I know when I was living with a girl in college, some issues we had to discuss were:
- Schedules, one may go to bed early and wake up early and the other may go to bed late and wake up late - discuss getting ready for bed/class the next day, etc.
- Fridge, is everything fair game or only use what you buy
- Common space, rules/expectations
- TV, sleeping with it on or off
- Study Habits, study in the room or study outside the room in the library or elsewhere
Remember, they are going through the same thing learning to live with you. If tension arises down the road, I would personally deal with it immediately. If in the future things bother you, take time to understand why it's bothering you, is it bothering you due to stress/tiredness or is it a true issue, and then discuss with them in a calm matter.
Also, understand people handle stress differently. I know my roommate and I weren't as close during finals, big tests, sporting events (my roommate played softball for the University), or when we went home from breaks. We both had a tendency to kind of "shut down" in readiness to go home and have our own space again, since we both were several hours from home.
I hope this helps!