Depression (mood)

In the fields of psychology and psychiatry, depression refers to a state of low mood and aversion to activity. While most often described as a disease or dysfunction, there are also strong arguments for seeing depression as an adaptive defence mechanism.

The Diagnostic and Statistical Manual of Mental Disorders defines a depressed person as experiencing feelings of sadness, helplessness and hopelessness. In traditional colloquy, "depressed" is often synonymous with "sad", but both clinical depression and non-clinical depression can also refer to a conglomeration of more than one feeling.

Biological influences of depression are varied, but may relate to malnutrition, heredity, hormones, seasons, stress, illness, drug or alcohol use, neurotransmitter malfunction, long-term exposure to dampness and mold^[1], back injury, and to aerosol exposure. There are also correlations between long term sleep difficulties and depression. Up to 90% of patients with depression are found to have sleep difficulties.
http://en.wikipedia.org/wiki/Depression_(mood)