Answer to Question 1
The most prominent feature of depression is dysphoria, or feeling down and blue. The second major component of clinical depression is physical symptoms. These include insomnia, changes in appetite, diffuse pain, headaches, and fatigue, among others. The third diagnostic criteria is that the symptoms must last for at least two weeks. This is to rule out any transient negative symptoms that are common to all adults. Fourth, other causes for the observed symptoms must be ruled out. Other causes include such things as other health problems, neurological problems, medications, and psychopathologies. Finally, the symptoms must be evaluated as to how they are affecting the person's life. In other words, do the symptoms cause significant impairment in everyday life?
Answer to Question 2
The idea behind cognitive behavioral theory is that experiencing unpredictable and uncontrollable events leads to a feeling of helplessness. This feeling leads to depression. In addition, perceiving the cause of negative events as something permanent and pervasive about the self also plays a role in feeling helpless and hopeless. People tend to ruminate on these negative ideas, which can lead to sleeplessness. Some have argued that negative thoughts and ruminating are linked to an inappropriate setting of the pleasure experience detector in the brain.