For me it's mood, energy, and sleep.

One of the doctors where I work doesn't consider it a mood disorder because serotonin isn't the primary neurotransmitter involved, and dopamine is. He thinks bipolar depression is different from unipolar depression. He says unipolar depression is a lack of serotonin. He thinks bipolar depression is caused by overusing and using up dopamine during mania/hypomania, especially because of staying awake. So you have a ton of dopamine, and you are energetic and elevated, and then it's all used up and there is a crash with a lack of dopamine, no energy, no pleasure, and depressed mood. He doesn't consider it a mood disorder, he considers it a disorder of repetitive behavior using up dopamine. I agree partially. I think the dopamine theory makes sense, but I definitely think it's a mood disorder. I don't think you need primary serotonin problems to have disturbed mood. He doesn't know I'm bipolar, so I couldn't really challenge him on his theory, but I don't think the conclusion that he is drawing makes much sense. Anyways, my point is that staying awake and high energy states seem to cause the low energy depressive crashes, at least some of the time.