(And probably other kinds of rhythms, too.)
A long time before I was diagnosed with bipolar disorder I figured out that I felt a whole hell of a lot better if had a regular schedule. Not just sleep schedule, but work schedule, and regular doses of socialness instead of nothingnothingnothing and then hoursandhours of seeing other people.
Circadian rhythms are important in bipolar disorder; lithium is involved in helping reset circadian rhythms. But we have other rhythms, too. Social rhythm disruption often shows up in bipolar disorder, when people swing between the gregariousness and over-gregariousness of hypo/mania, and the withdrawal of depression. And externally regulated social rhythms (being around and not around people on a regular schedule) can help regulate bipolar disorder. (Not all on their own, duh, nor for everyone, but they can be helpful.)
Around The Clock is a blog written by a circadian rhythm and photoperiodism researcher (in case you were wondering, as I was, photoperiodism is “the physiological reaction of organisms to the length of day or night“). He has an entry with several short pieces on circadian rhythms, social rhythms, and bipolar that I recommend. I do want to say that he overstates the effect of social rhythm disruption on people with bipolar; it is not true that any new relationship, job, etc., will necessarily result in wild cycling. But don’t let that detract from the general interestingness of the post.