I know, I know, you neeeeeed to Facebook your friends for three hours every night. Well, actually, you don't. Yes, it is important to have social relationships, but you really don't need to be texting or chatting until the wee hours.
Cross-country is very, very, VERY demanding on your body. That means you really need to get enough rest every single night. Of course, you will also have homework and maybe a piano lesson or something, too.
Research shows (yes, I'll find the published study I'm thinking of when I have a chance) that you eventually have to catch up on your sleep. That's YOUR sleep, which may be different than someone else's. Let's say that YOUR body needs 9 hours of sleep a night. If you only get 8 hours of sleep on Sunday, Monday, Tuesday, Wednesday, and Thursday nights, that means you need an extra 5 hours on Friday night to catch up. If you don't catch up over the weekend (which you might not be able to do), you will fall further and further behind.
Falling behind on your sleep can have very negative consequences on the really important stuff, like your mood and your schoolwork. It can even make it easier for you to catch colds. But it will also significantly affect your performance in cross-country.
It's not that hard to figure out how much sleep you need - just track how many hours you get a night and how you feel the next day (Hint: you will probably need more during XC season than at other times). I am willing to bet you are probably smart enough to come up with a number of hours for yourself. Once you do, stick to that. Sure, you may have an occasional night or two that you don't get enough, but do your best to not fall behind.