sometimes you're anxious and don't realize it. When I was teaching a group of sixth graders years ago I had a horrible case of insomnia. I didn't think I was stressed at the time, but looking back I realized they were one of the WORST group of students I had ever had and it was taking its toll on me mentally.
Try laying still and resting even if you're not sleeping. I know it gets so boring but you don't want to get up and do something stimulating like watch TV or get on the computer. You might try a glass of milk or a banana-- maybe you're hungry. I woke up a lot in the night during my pregnancy. I found if I had a snack it helped me go back to sleep faster. If you can, try taking an evening walk and then a warm bath to help relax you. Don't worry-- it's temporary.