Transcript: Most people know what depression is - almost everyone suffers from occasional depression with symptoms like sadness, pessimism and low energy.  It is only when depression lasts two weeks or more that it becomes a clinical disorder, at which point it is called Major Depression.  People sometimes have a hard time understanding the difference between depression and normal sadness.   It is important to understand that when someone has depression there are actually physical changes in the structure of the brain as well as reduced levels of important chemicals called neurotransmitters.  Neurotransmitters are chemical messengers that allow communication between nerves, but they are also important for mood regulation.  People suffering from depression typically have reduced levels of neurotransmitters, especially serotonin.   Lower levels of serotonin lead to mood destabilization and depression. People suffering from depression can also have related changes in their brain structure.  For example, people with a history of depression have a smaller hippocampus than others.  This is important because the hippocampus is an important serotonin receptor.Although people commonly think of depression as a single illness, there are actually many different types, with different causes and treatments.  One common type of depression is Major Depression.   Symptoms often include overwhelming feelings of sadness, loss of interest in enjoyable activities, low energy and feelings of worthlessness. It may also result in poor sleep, appetite changes, and negative thinking.  Another kind of depression, Dysthymia, is characterized by a chronic lack of pleasure in life.  Its symptoms are less severe than major depression, but Dysthymia tends to last for long periods of time. Adjustment depression disorder can occur in the aftermath of a sad or traumatic event.  A period of unhappiness is normal, but if the depressed feelings continue for several months then it is called adjustment depression disorder.  One other common kind of depression is Seasonal Affective Disorder, a pattern of depression related to a lack of exposure to sunlight.   Typically, SAD sufferers notice symptoms during winter, when days are shortest, and can often be helped with a light box that replaces lost sunlight.  Depression is really a catch-all term for many related illnesses.  Excellent treatments exist for most kinds of depression, but self-diagnosis is tricky.  If you think you are suffering from depression, the first step towards feeling better is to see a doctor.   If you are interested in learning more about this topic, check out other videos and sources on this subject.