Depression is a complex mental health disorder that is thought to be caused by a combination of genetic, biological, environmental, and psychological factors. The exact cause of depression is not known, but research suggests that certain neurotransmitters, such as serotonin and norepinephrine, may be involved. Additionally, it is believed that changes in the structure and function of certain brain regions, including the hippocampus, can lead to depression. Studies have also suggested that disruptions in circadian rhythms, or the body's internal clock, may also contribute to the development of depression.