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