That said, I think only in their extremes are mania and depression actually unintelligible to ordinary folks. That is: At their utmost intensity, they are unlike anything a normal person ever experiences (mania is, in particular, qualitatively different at the end than any healthy mood state), but at most times, they are not at all different from the maximally intense moods everyone knows—just more so, longer-lasting, and disconnected from normative causes.
To understand what having bipolar disorder "feels like," keep in mind the following:
First, bipolar is less about short-term mood instability than about long-term mood cycles, which can last months, years, or in rare cases even decades. (See F.M. Mondimore for more on cycles and durations.) Instability is part of it, but not the only part.
Second, the cumulative effect of these cycles on the formation of a personality is significant. After a childhood of radically changing interests and attitudes on such a timeline, one develops a certain excitability, flightiness, distractibility, or perhaps that's just me. But this is a major part of bipolar: the personality that is shaped by a lifetime of intense, fluctuating moods.
