While the best known symptoms of bipolar disorder are mood related (and the disease itself is a mood disorder), bipolar disorder affects a person in many other ways as well.
When people with bipolar disorder experience highs or lows, they experience problems with overall cognitive functions as well as mood. A person may one day have a razor-sharp mind and sharpened intellect, and the next day have muddled thoughts and a sluggish thought process.
It also messes with sleep patterns. While experiencing a "high," someone with bipolar disorder won't sleep as much (sometimes hardly at all), and seemingly won't be the worse for it during the day. In fact, lack of sleep is often a precursor to a manic episode that hasn't presented itself yet. When experiencing a "low," a person will oversleep and never feel fully rested and alert.
Highs and lows also contribute to bad lifestyle choices like smoking, drinking, poor diet and drug use.