Fragmented

For some reasons, I am unable to sleep well at night. Doctors call it "fragmented sleep". I am tired and I fall asleep easily. But I would wake up in the middle of the night and would have trouble falling sound asleep again. I wish I knew what is the reason behind this. Stress? Maybe. My "Fragmented sleep pattern" only occurs during weekdays, when I need to wake up early to go to school. During the weekends, I usually do not have such problems.