There have been some great replies here and a lot of useful information.

I have to agree with Nammu, that very very short mood changes are likely the product a personality disorder, specifically -usually- BPD, and not Bipolar Disorder.

BP requires a persistent mood, lasting at least one week (I can't remember exactly how long, but that, or more). There are, and I think they are few and far between, pdocs who believe in ultra rapid cycling, but I cannot concur.

Mood changes within hours and within a given day, I think, are often triggered by one's environment, even if it's not recognized by the sufferer as such. This is an indication of BPD, but even more so, if those triggers are related to how one relates to other people/relationships with others.

This might not be a popular opinion, but it's what I believe. The DSM indicates this, and even if you're skeptical of the DSM, to me it just makes sense. Pdocs need to take the time with patients to determine their patterns of relating to others and if 'mood swings' correspond to this kind of relating (fear of abandonment, etc.). Patients, also, need to have the insight (often through therapy) to see these patterns for what they are and try to eliminate them. I'm not criticizing anyone for lack of insight, but I think that it can potentially take many years to figure out how one is relating to others, and how one's mood changes according to how one relates to others.