Sleep has a lot to do with it, and also light and dark. I only get manic between March and July, and then usually a lot of St. John's Wort is involved, and then I just get hypomanic - enough that I can't keep up with my own thoughts and I try to do a lot more at one time than is reasonable. But anyway, the days getting longer and spending more time in the sunshine are major factors.

One thing that can be effective in managing bipolar disorder is "dark therapy." http://www.ncbi.nlm.nih.gov/pubmed/17637502

The idea is to eliminate exposure to light, at least the blue lightwaves, during the evening and night hours. It also can help to establish a regular routine and stick to it, including bedtime and regular meals, to keep your circadian rhythm on track. Log your mood, bedtime and wake time, medications, activity level, and anything else that you think might be relevant, and watch for patterns. Decreased sleep is a definite warning sign.