Mirtazipine (I probably spelled it wrong but oh well) for insomnia?

I talked to my doctor because I was unable to fall asleep at night. I would lie there for 2 or 3 hours (sometimes more) with work related thoughts and plans buzzing in my head unable to fall asleep. He prescribed mirtazipine. It seems to work, but when I try to read up on it, everything I see seems to indicate that it is used to treat depression. Is it normal to use it as a sleep aid? I am confused.