I can't stay asleep... who should I go to?

I'm a 29 yr old woman whose having trouble staying asleep for at least 8 months or so. I usually have a very "light sleep", and can easily wake up, but for the past months, I wake up after 3-4 hours of sleep and I can't go back. If I try to get some sleep, I wake up every hour after that. Because this, I'm always tired, easily irritable and can't focus at work as I used to and I'm having trouble loosing weight. Even the days that I'm extremely tired, I still can't have a full night sleep. I don't have TV or any other stimulants in my room and I even bought a new mattress, but this don't seem to work either.

What doctor should I go to for evaluation?

Thanks.