The exact cause of bipolar disorder is unknown. Experts believe there are a number of factors that work together to make a person more likely to develop it.
These are thought to be a complex mix of physical, environmental and social factors.
It's thought bipolar disorder is linked to genetics, as it seems to run in families.
The family members of a person with bipolar disorder have an increased risk of developing it themselves.
But no single gene is responsible for bipolar disorder. Instead, a number of genetic and environmental factors are thought to act as triggers.
A stressful circumstance or situation often triggers the symptoms of bipolar disorder.
Examples of stressful triggers include:
- the breakdown of a relationship
- physical, sexual or emotional abuse
- the death of a close family member or loved one
These types of life-altering events can cause episodes of depression at any time in a person's life.
Bipolar disorder may also be triggered by:
- physical illness
- sleep disturbances
- overwhelming problems in everyday life, such as problems with money, work or relationships
Chemical imbalance in the brain
There is some evidence that bipolar disorder may be associated with chemical imbalances in the brain.
The chemicals responsible for controlling the brain's functions are called neurotransmitters, and include noradrenaline, serotonin and dopamine.
There's some evidence that if there's an imbalance in the levels of 1 or more neurotransmitters, a person may develop some symptoms of bipolar disorder.
For example, there's evidence that episodes of mania may occur when levels of noradrenaline are too high, and episodes of depression may be the result of noradrenaline levels becoming too low.