The difference between bipolar disorder and unipolar disorder ("normal" clinical depression) is that bipolar disorder involves both elevated and depressive mood states. How long each state lasts, and how intense or obvious it is, varies widely from person to person.
Fluctuating from one mood state to the next is called "cycling", and can either improve or impair how you function depending on the severity. Changes may be seen with changes in your energy level, sleep patterns, activity level, social rhythms and cognitive functioning. During these times, some people may have difficulty functioning.
People with bipolar disorder usually have three times as many depressive episodes as manic phases.
Sometimes people experience both depression and mania at the same time. This is called mixed state. This can be more dangerous than any other cycle, as the combination of hyperactivity and depression create increased likelihood of alcohol or substance abuse, and suicidal tendencies.