I think theama is right. I messed up on the definition of a mixed episode and rapid cycling. Also that the depressive phase has to last at least 2 weeks.

But yeah, the spectrum of bipolar is tricky and all kinds of moods are possible, and none of them will necessarily stick around nonstop for a certain period. They'll do whatever they like, but the point is, your mood is not "normal" and it changes at some point when you're bipolar. You may transition from depressed to manic through a mixed stage or not. You may go from hypomanic to either manic or down to depressed. The DSM-IV relies on self reporting, so if people in the depressed phase don't recall a hypomanic time during the two weeks of depression, there's more evidence that 2 weeks is the magic number. Same goes for people in a manic episode for a week or more. I'm sure many of them have had depression or mixed times within their mania, but the mania is the most pronounced, so that's what they talk about.

Just a thought.