Sadness

Depression, or, more properly, a depressed mood, refers to a state of non-clinical melancholia that is shorter than 2 weeks in duration and distinctly differentiated from a diagnosis of clinical depression. A depressed mood is generally situational and reactive, and associated with grief, loss, or a major social transition. A change of residence, marriage, divorce, the break-up of a significant relationship, graduation, or job loss are all examples of instances that might trigger a depressed mood.

In the field of psychiatry the word depression can also have this meaning but more specifically refers to a mental illness when it has reached a severity and duration to warrant a diagnosis. The Diagnostic and Statistical Manual of Mental Disorders (DSM) states that a depressed mood is often reported as being: "... depressed, sad, hopeless, discouraged, or 'down in the dumps'."

In a clinical setting, a depressed mood can be something a patient reports (a symptom), or something a clinician observes (a sign), or both.

Determinants of mood
Depression can be the result of many factors, individually and acting in concert.

Environment
Reactions to events, often a loss in some form, are perhaps the most obvious causes. This loss may be obvious, such as the death of a loved one, or having moved from one house to another (mainly with children), or less obvious, such as disillusionment about one's career prospects. Monotonous environments can be depressing. A lack of control over one's environment can lead to feelings of helplessness. Domestic disputes and financial difficulties are common causes of a depressed mood. Love, or lack of being able to express your feelings can lead to a feeling of unexplainable sadness or grief.

Psychological Factors
Sometimes the depressed mood may relate more to internal processes or even be triggered by them. Pessimistic views of life or a lack of self-esteem can lead to depression. Illnesses and changes in cognition that occur in psychoses and dementias, to name but two, can lead to depression. Depression may also be comorbid with cardiovascular disorders. Some examples are Toronto Maple Leafs fans reacting to a losing steak by their team. .

Physiological Considerations
The etiology of depression is still being actively investigated. Because different biologicals variables seem to ellicit similar expressions of depression, it is difficult to precisely pinpoint the condition's root. Some general physiological considerations include  Genetics (i.e. an acquired disposition to depression), neurochemistry (e.g. decreased Serotonin release), hormone imbalance (e.g. PMS in women), illness and seasonal factors. See Clinical depression.

Adaptive benefits of depression
While a depressed mood is usually seen as deleterious, it may have adaptive benefits. The loss of a loved spouse, child, friend or relation, a physical illness or loss of lifestyle, tends to lead to feelings of depression. Freud noted the similarities between mourning and depression (then called melancholia) in a now famous paper entitled, "Mourning and Melancholia". The depressed mood is adaptive in that it leads the person towards altering their thought patterns and behavior or way of living or else continues until such a time as they do so. It can be argued that depression and clinical depression is in fact the refusal of a person to heed the call to change from within their own mind. For example, in mourning it is essential that one must eventually let go of the dead person and return to the world and other relationships.

Depression appears to have the effect of stopping a person in their tracks and forcing them to turn inwards and engage in a period of self reflection; it is a deeply introspective state. During this period, which can last anything from days to years, the individual must find a new way to interpret their thoughts and feelings and reassess the extent to which their appraisal of their reality is a valid.

Seasonal affective disorder may point to an atavistic link with behaviour in hibernation.