Bipolar Disorder affects nearly six million American, typically resulting in extreme shifts in mood, adversely affecting sleep and behavior patterns. But what exactly causes Bipolar Disorder? The exact cause of Bipolar Disorder is not completely understood. However, over the past decade, experts' understanding of this mood disorder has increased, leading to the belief that a number of factors including biological, genetic and environmental components may be responsible for causing Bipolar Disorder. Current thinking places considerable emphasis on biological causes, based on the belief that people who suffer from Bipolar Depression may have an imbalance in important brain chemicals called neurotransmitters, such as norepinephrine, serotonin and dopamine, each of which plays a role in mood and emotions. For example, when certain neurotransmitter levels are low, depression may result. Similarly, abnormally high levels of some neurotransmitters may cause mania, which is characterized by either sudden euphoria or rage, along with hyperactivity, decreased need for sleep and reckless behavior. Experts believe these causes may lie dormant indefinitely, and could be activated for an unknown reason, or be triggered by various factors including a traumatic life event such as abuse, the death of a loved one, job loss or divorce. It's also thought that stress, lack of sleep or frequent use of stimulants may prompt the onset of Bipolar Disorder, for those who are vulnerable to the illness. Imaging studies suggest brain structures of those with Bipolar Disorder sufferers may differ slightly from those who don't have the illness. Scientists have also observed abnormal hyperactivity in areas of the brain associated with emotion, and low activity in areas related to concentration, judgment and inhibition. Other imaging scans and tests have resulted in researchers observing over-secretion of the stress hormone cortisol in people with Bipolar Disorder, as well as an excessive influx of calcium into brain cells. Numerous studies suggest Bipolar Disorder has a significant genetic component. Researchers have linked multiple genes involving several chromosomes to the illness. Approximately half of Bipolar Depression sufferers have a family member with a mood disorder. Having a parent with Bipolar Disorder may pose a 15-25 percent risk of developing the illness; having an identical twin with the disorder may increase risk to more than 85 percent. Scientists are also examining links between certain viruses and Bipolar Disorder. Researchers are looking into possible connections between the illness and the Borna disease and Epstein-Barr viruses. And some evidence suggests an increased risk for adult children of mothers with herpes simplex 2 during pregnancy. The good news is Bipolar Disorder is treatable and sufferers can lead full and productive lives. If you think you may be suffering from Bipolar Depression, please see a mental health professional.