I suffer from severe insomnia practically every summer. For me, it typically corresponds to a period of high volume training. Even though I'm working my body very hard, and walking through most of the day almost as a zombie, once I hit the pillow I am simply incapable of falling asleep and/or staying asleep. I did 140 miles one week this past summer averaging maybe 4 hours of actual sleep each day, probably less. Of course, I ended up crashing and burning a few days after that.
I think two things are at work when I suffer from insomnia. One, the high mileage means the endorphines are really flowing, which makes it hard for your body to "wind down" at night. Second, as it turns out I was suffering from anemia, which can also lead to difficulty sleeping. Basically, I was not maintaining adequate nutrition to support the training I was trying to do. This really screws with your metabolism and hormones, which can produce insomnia.
So I would take a good look at your training and diet, and make sure you're not doing anything that is placing a sudden, increased demand upon your body.