I just happened to find this:

Although popular culture tends to equate mania with happiness and depression with sadness this isn't really the best way to think about what is happening in bipolar disorder. Bipolar Disorder involves not so much a swing between happy and sad states, as it does a swing between high and low energy states. When in a high-energy state, people appear happy because they are motivated and excitable, whereas in a low energy state, people feel sad, and lack motivation and enthusiasm. As the energy level of a manic episode increases, the early happy mood tends to degenerate into a more agitated and psychotic state which may be experienced more as terrifying than happy, but which is nevertheless very energizing. Similarly, as a depressive mood state increases, people may go from merely feeling badly about themselves to literally not being able to leave their bed. Thus, the happy and sad moods that are thought to characterize mania and depression respectively are results of different energy states and not necessarily primary features of the disorder.

Here's the link to the article. I am just reading it now:

http://www.mentalhelp.net/poc/view_d...&id=11193&cn=4