Depression is caused by imbalances in the chemicals that help to send messages in the brain. These chemicals in our brain also help to regulate our emotions, behavior, and thinking. Depression is also thought to have a strong genetic component (i.e., family history). While the tendency to be depressed may be inherited, the onset of depression can be provoked by many factors.
For example, depression can be triggered by certain medical conditions such as stroke, long-term pain, sleep disorders such as insomnia or sleep apnea, or post-traumatic stress disorder (PTSD). Other conditions such as hypothyroidism, if left untreated, can lead to symptoms of depression.
Feelings brought on by certain conditions such as erectile dysfunction may also eventually lead to depression.
Some women who have recently given birth may be susceptible to postpartum depression. Seasonal affective disorder (SAD), an extreme case of the "winter blahs," is estimated to affect millions of Americans.
Those who are diagnosed with bipolar disorder, also known as manic depression, experience mood instability characterized by manic peaks with symptoms that include heightened energy followed by valleys of depressive episodes.