Judging from your post, one would assume you are in high school. Why not just say your parents grounded you from the phone during the night because they feel too much talking on the phone is affecting your school grades? Don't give more information that is neccessary to avoid her catching your lie, but let her know it's affecting your school performance, and you should stop, or limit the times she calls you per week.
Once you have developed a continuous pattern (precedent) of talking on the phone daily at night for hours, it's often very difficult to remove yourself from the situation. That's why it's best to ensure you are on a pedastal from the beginning of a relationship to ensure none of this happens, that the initial spark remains alive between you and that special girl, and to ensure she remains a part of your life, NOT your life.
Best bet is to gradually slip away from her in terms of "picking up" your phone. One day, just don't bother to pick up your phone. Next day, in school, she will ask you why you didn't pick up and just let her know that you were super busy. If you played your cards right from the beginning, she won't over react to the issue, pick up, and leave. Keep doing this, maybe once this week, twice next week, etc. She will sooner get the message - trust me! Remember, it doesn't hurt not to pick up her calls once in a while. The anticipation to talk to you the next day will be enormous if you stop picking a few of her calls! You are making yourself WAY too available to her, somebody needs to get off that whip!
Now, there is an alternative method you can use. However, I'd only use it if you and the girl in your post are in a LTR and there is a special intimacy between you two. Most relationships fail because of a lack of communication. If you feel that you and this girl communicate effectively with each other, just tell her the HONEST truth. Let her know that you have other things to do with your time, and you can't stay up talking to her because you have school the next day! Oh, and if there isn't good communiation between you two, be prepared to answer back to comments such as: "Oh, so you don't want to talk to me anymore?" and "If you wanted to ignore me, you could have just said so." I'll pray, for your sake, that this doesn't happen.
Best of luck - let us know what you decide to do!
