By Mills Baker, Head of User Experience, Giftly
I have bipolar disorder, as does my mother and as did her mother. I am the sort of person who "seems" bipolar to people; that is, my energy, creativity, instability, mercuriality, and easy gregariousness confirm many of the popularly-imagined stereotypes about bipolar people.
That said, I think only in their extremes are mania and depression actually unintelligible to ordinary folks. That is: at their utmost intensity, they may be 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 not at all different from the maximally-intense moods everyone knows: just more intense, longer-lasting, and disconnected from normative causes.
To understand what having bipolar disorder "feels like," keep in mind the following:
- Bipolar is less about short-term mood instability than about long term mood cycles which can last months, years, 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.
- 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.
- Cycles grow in intensity over time. This means that at first in mania, for example, you're simply in a great mood; then you're really in an extraordinarily creative, kinetic, charming mood; then you're the life of all parties and you're feeling pretty libidinous; then you're doing irresponsible things and fleeing a pursuing psychosis; then you're in psychosis, tortured by acousticovisual hallucinations, paranoia, and your own penchant for completely unacceptable reactions and behavior. This progression can take days, weeks, months, or years.
- The same progression tends to hold for depression.
Kay Redfield Jamison  is a psychiatrist at Johns Hopkins who suffers from, treats, and writes about bipolar disorder. About mania, she writes:
The ideas and feelings are fast and frequent like shooting stars, and you follow them until you find better and brighter ones. Shyness goes, the right words and gestures are suddenly there, the power to captivate others a felt certainty. There are interests found in uninteresting people. Sensuality is pervasive and the desire to seduce and be seduced irresistible. Feelings of ease, intensity, power, well-being, financial omnipotence, and euphoria pervade one's marrow. But, somewhere, this changes. The fast ideas are far too fast, and there are far too many; overwhelming confusion replaces clarity. Memory goes. Humor and absorption on friends' faces are replaced by fear and concern. Everything previously moving with the grain is now against-- you are irritable, angry, frightened, uncontrollable, and enmeshed totally in the blackest caves of the mind. You never knew those caves were there. It will never end, for madness carves its own reality.
About depression, she writes:
In its severe forms, depression paralyzes all of the otherwise vital forces that make us human, leaving instead a bleak, despairing, desperate, and deadened state... Life is bloodless, pulseless, and yet present enough to allow a suffocating horror and pain. All bearings are lost; all things are dark and drained of feeling. The slippage into futility is first gradual, then utter. Thought, which is as pervasively affected by depression as mood, is morbid, confused, and stuporous. It is also vacillating, ruminative, indecisive, and self-castigating. The body is bone-weary; there is no will; nothing is that is not an effort, and nothing at all seems worth it. Sleep is fragmented, elusive, or all-consuming. Like an unstable gas, an irritable exhaustion seeps into every crevice of thought and action. 
Jamison is well known, too, for her research on the link between bipolar disorder and creativity, which leads me to my conclusion:
To know what it "feels like" is to know the qualia, the phenomenological experiences, a bipolar person encounters which you do not. I don't think there are many of these. Going berserk, being creative, having an awful temper, not being able to trust my own emotional reactions: these have a certain weight when I list them out, discuss them as little tragedies; they sound unique!
But everyone loses it; everyone has their moments of charisma, creativity, success, strength, achievement; and everyone struggles with themselves. You may not hallucinate, but I bet you can understand what it's like for your mind to misbehave, react insanely. If you haven't yet lost control of yourself in life, wait.
Bipolar people have a tendency to comfort ourselves by saying that our more intense experience of typical phenomena constitutes an election: we are elite, more intense, deeper! Jamison's own excellent research on bipolar artists has amplified this: the popular Western conflation of insanity, artistic talent, and melodrama permits a kind of sentimental self-regard: Yes, I'm crazy, but I'm also probably in some difficult-to-establish-way deeply brilliant!
Perhaps this is true for some, but it seems mostly to me to be a consolatory story, the sort of inversion that Nietzsche describes: this illness is really a kind of health, a kind of deeper seeing. I like being bipolar, like my life a lot, but I am uncomfortable with this persistent meme, largely because I'm sometimes confused into believing it myself. Indeed: bipolar sometimes seems to me like its chief symptom is that a patient confuses himself with an artist.
(Or more generally: an exception. Mondimore notes that throughout history, "grandiosity" has changed in its expressions. An important symptom of bipolar, grandiosity once expressed as women saying they were pregnant with kings or the messiah, men believing they were kings or the messiah; presently, our insanity is more equal: we all believe we special exceptions of one sort or another).
Last: I've been in treatment now for almost twelve years, on the same cocktail of medications for years and years. For me, the most enduring way that bipolar "feels" different is in how I cannot trust my reactions. When someone says something to you and you recognize it as an insult, as abuse, your reactive anger is appropriate and you can commit to it; or you can make some determination based on your values, your reason, and choose a different course of action. I can't even trust that the person insulted me. I can't trust my emotional perceptions or reactions.
That's the strangest thing about how it feels, after the dust of the actual disorder settles, a decade in: the open insanity has ended, the idea that I'm a secret poet is absurd, and what I have left is a more or less normal life (sans drinking, crucially) in which I have to emphasize "mental hygiene" (prioritizing regular sleep, for example) and in which I always feel doubt about what I think and feel, as we all probably should anyway.
(As Anne Zieger helpfully noted, this answer is not fully comprehensive: phenomena such as rapid-cycling, which I've experienced only from SSRIs, aren't discussed, and the variations of bipolar disorder aren't either. How it "feels" to be bipolar naturally varies widely from person to person, just as how it feels to be sane must, so I should emphasize that this answer is how it feels for me alone. Thanks!)
(I haven't read Jamison in years; apologies for plucking quotes from Goodreads!)More questions on mental health: