Oliver Sacks in the NYRB:
The special qualities of mania have been recognized and distinguished from other forms of madness since the great physicians of antiquity wrote on the subject. Aretaeus, in the second century, gave a clear description of how excited and depressed states might alternate in an individual, but the distinction between different forms of madness was not formalized until the rise of psychiatry in nineteenth-century France. It was then that “circular insanity” (folie circulaire or folie à double forme)—what Emil Kraepelin later called manic-depressive insanity and what we would now call bipolar disorder—was distinguished from the much graver disorder of “dementia praecox” or schizophrenia. But medical accounts, accounts from the outside, can never do justice to what is actually experienced in the course of such psychoses; there is no substitute here for firsthand accounts.
There have been several such personal accounts over the years, and one of the best, to my mind, is Wisdom, Madness and Folly: The Philosophy of a Lunatic by John Custance, published in 1952. He writes:
The mental disease to which I am subject is…known as manic depression, or, more accurately, as Manic depressive Psychosis…. The manic state is one of elation, of pleasurable excitement sometimes attaining to an extreme pitch of ecstasy; the depressive state is its precise opposite, one of misery, dejection, and at times of appalling horror.