SWEDE!
This may help ..... (I did not write it though. I just copied and pasted it!
It could be stress, which thoughts are keeping you mentally alert at all times. Or it could be an external thing: bedroom/sleeping environment, physical activity during the day, what you consume through the day, if you develop a routine in the evening. All these things could contribute to the quality of sleep you're getting. So, your sleeping environment: it should be cool, but not uncomfortably so. Comfortable with PJs and covers on. It should be calming colors like blue, brown, cream, marroon, soft orange. Also, try some kind of "white noise" like clear radio static or put a little fan in your room. Next, try to exercise at least 30 minutes a day, and not more than 2 hours before bed. Your body may still be running on adrenaline 2 hours after your exercise. Then, what about what you're eating? No eating after dinner, if you can help it. And, if you can develop a bedtime routine, it will help put you into a mental flow of "now it's time for bed". Also, if on big test days, or big event days, it's okay to take a OTC sleep aid, but try not to take it everyday. If you feel like stress is the cause, exercise, journal, watch a funny tv show before bed, talk to a loved one on the phone before bed. But, if instincts tell you that you have something else going on, listen.