Almost all of us have known people whose moods can change from calm to stormy, warm to cold and bright to dark. For most of us, these transformations are part of the natural rhythm of life—normal responses to everyday events and emotions. Some people, however, lose the ability to modulate their moods in a balanced way. At times they become flooded with a heightened sense of euphoria. At other times they feel like they're drowning in a sea of depression. In some instances these feelings alternate in quick succession. If these signs describe someone you know, he or she may be suffering from bipolar disorder.