I cant get the 8 hours of sleep?

For some reason, I cannot stay asleep any longer than 7-7.5 hours. The last time I tried to sleep for 8 hours, I woke up before. This was just a few weeks ago. I don't know, I just feel energized before 8 hours. I am a 13 year old girl.I do not have trouble falling asleep and getting up. What is wrong here? Am I getting insomnia on my last hour/half hour?