It would be wrong to say that bipolar moods are purely a matter of cycles of brain chemistry. We all know that triggers have a lot to do with changing or worsening our moods.
Triggers can be life-changing events such as the death of a loved one or friend, a marriage, buying a new house, losing a job. Such events - good or bad - are really stressful. A bride might be hypomanic before her wedding and through her honeymoon, then become depressed when she has to return to everyday living. While buying and moving into a new home might send a person manic, the stress of selling the old house first (especially in today's market!) might be a trigger for deep depression (that happened to me).
Triggers can be less dramatic, too. Changes in weather can have an effect on moods, and I'm not even speaking about Seasonal Affective Disorder - just, for example, a long series of gray and gloomy days can lead some people into depression and irritability.
Pain can be a trigger. Many people with bipolar disorder also suffer from fibromyalgia or migraines. An illness or injury can whack a hypomanic mood down quickly. I've had two serious back injuries in the last year and both sent my mood down and down very quickly.
Sometimes even a chance remark by an acquaintance can make a difference. You go to a party thinking you look fabulous, and you overhear someone saying, "Gosh, she looks ridiculous in that dress." Then you start second-guessing yourself, talk trash to yourself about your weight, think everybody is talking negatively about you and soon your mood has changed.
Maybe you get a raise, a promotion or a bonus and that makes you think you can splurge with that extra money. You'd been feeling depressed, but now you're suddenly excited and giddy, and you go out and spend far more than what you received in a manic spending spree.
A couple of my small triggers that can lead to or worsen depression are being criticized (whether fairly or not) and being laughed at (a holdover from my childhood as a target for teasing). Getting extra money is a hypomanic trigger. What triggers you?