My flatmate is very inconsiderate. What can I do?

Maybe try to talk to him and tell him, you know, you have to sleep to every once in a while. If he has to make a phone call at 3 in the morning, he can try to be a little more quiet about it since you were trying to sleep. If he doesn't stop, maybe invest in a good pair of earplugs. But seriously, I would talk to him about it and try to make him understand that he's not being very fair by waking you up and everything.