This is not the end-all, but PLEASE begin by looking at her (and your) diet. If it's mostly processed foods w/artificial colors and/or flavors, she is not getting the nutrients she needs that help to stave off things like depression. B vitamins, iron, and folic acid are key players as are omega-3 fatty acids.
Aerobic exercise has been proven to be just as effective at helping with depression as well.
I think imminent puberty and the accompanying hormonal changes are a factor as well. I'm glad to hear she's getting enough rest. Make sure all of your dairy is marked rBST / rBGH free, so she isn't effected by the bovine growth hormones. Check your milk labels.
I definitely experience lots of highs and lows due to hormones (even to the point of PMDD at one time) and all of the above considerations helped me tremendously. Dep drugs are so dangerous - esp for adolescents - and can open you up to a host of other health issues. I want to encourage you to take these natural steps first - esp with your daughter.
Again, I know it isn't the end-all, but the vast majority of Americans greatly underestimate the effects of the foods (or non-foods) they eat regularly. Doctors know VERY LITTLE about preventive nutrition and the healing power of foods. You'll have to do some of your own research here.
These are some of my favorite resources:
http://www.100daysofrealfood.com/
http://feingold.org/ (specifically mentions ADHD, but good advice all-around)
http://www.naturalnews.com/
http://www.foodrenegade.com/
Good luck to you both!