I agree; that experience sounds like a mixed phenomenon. It's quite common. I read an article once that challenged the idea of "pure" depression, claiming that all depressions (including those experienced by mostly unipolars) are partly mixed!

I can completely relate to being physically exhausted but mentally wide awake. When I'm in that state, I feel like my head is that annoying person at every slumber party who just won't shut up and let everyone else sleep.

http://www.psycheducation.org/depression/Waves.htm

That might explain mixed states a little better. It's not the article I mentioned earlier, but it's very good IMO.