Depression (mood)

In the fields of psychology and psychiatry, the terms depression or depressed refer to both expected and pathologically chronic or severe levels of sadness, perceived helplessness, loss of interest or pleasure, and other related emotions and behaviours.

The Diagnostic and Statistical Manual of Mental Disorders (DSM) states that a depressed mood is often reported as feeling depressed, sad, helpless, and hopeless. In traditional colloquy, "depressed" is often synonymous with "sad," but both clinical and non-clinical depression can also refer to a conglomeration of more than one feeling. Such a mixture can include (but is not limited to) anger, fear, anxiety, despair, guilt, apathy, and/or grief, in addition to what many people would describe as typical "sadness". It is harmful for the human body and can affect proper functioning of the brain.

Biological influences of depression are varied, but can include malnutrition, heredity, hormones, seasons, stress, illness, neurotransmitter malfunction, long-term exposure to dampness and mold band 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

While a depressed mood is usually referred to (and perceived) as negative, it can sometimes be subtly beneficial in helping a person adapt to circumstance. For example, physical illness, such as influenza, can lead to feelings of psychological malaise and depression that seem, at first, only to compound an already unpleasant situation.
However, the experience of depression, or feeling "down," often results in physical inertia, which leads to the compulsion to rest. The fleeting helplessness and immobility of the physically ill may also serve to elicit care from others." From an evolutionary standpoint, some argue that depression could be at least partially related to atavistic fears that were originally based on real dangers. Paul Keedwell, in his book, How Sadness Survived: The Evolutionary Basis of Depression, suggests that, because "social support and interdependence were important features of the [human] ancestral environment," "the [peer] group could have offered extra help to the depressed person until the condition resolved."

Further, "...a depressed person may change the attitudes of other people around him, making them more sympathetic to his needs and therefore giving him a long term [social or reproductive] advantage."
Milder depression has been associated with what has been called depressive realism, or the "sadder-but-wiser" effect, a view of the world that is relatively undistorted by positive biases.