Bipolar disorder is a mental illness that causes dramatic shifts in a person's mood, energy, activity level and ability to carry out day-to-day tasks. The moods range from periods of extremely "up", energized behavior (known as manic episodes) to very "low", sad or hopeless periods (known as depressive episodes).
Symptoms and their severity can vary. A person with bipolar disorder may have distinct manic or depressed states but may also have extended periods - sometimes years - without symptoms. A person can also experience both extremes simultaneously or in rapid sequence.