Depression is a state of low mood and aversion to activity that can affect a person's thoughts, behavior, feelings and sense of well-being. Depressed people can feel sad, anxious, empty, hopeless, worried, helpless, worthless, guilty, irritable, hurt, or restless. They may lose interest in activities that once were pleasurable, experience loss of appetite or overeating, have problems concentrating, remembering details, or making decisions, and may contemplate, attempt, or commit suicide. Insomnia, excessive sleeping, fatigue, loss of energy, or aches, pains, or digestive problems may also be present.
Depressed mood is not always a psychiatric disorder. It may also be a normal reaction to certain life events, a symptom of some medical conditions, or a side effect of some drugs or medical treatments. Depressed mood is also a primary or associated feature of certain psychiatric syndromes such as clinical depression.
Life events and changes that may precipitate depressed mood include childbirth, menopause, financial difficulties, job problems, a medical diagnosis (cancer, HIV, etc.), bullying, loss of a loved one, natural disasters, social isolation, relationship troubles, jealousy, separation, and catastrophic injury.
Traumatizing events that took place in childhood can cause depression. Although childhood trauma and particularly child sex abuse is not always a factor of adulthood depression, it may create psychological pathways that can lead to depression. Research has been done in this field to demonstrate the chemical involvements explaining this phenomenon
Unequal parental treatment is also a risk factor.
Depressed mood can be the result of a number of infectious diseases, neurological conditions  and physiological problems including hypoandrogenism (in men), Addison's disease, Lyme disease, multiple sclerosis, chronic pain, stroke, diabetes, cancer, sleep apnea, and disturbed circadian rhythm. It is often one of the early symptoms of hypothyroidism (reduced activity of the thyroid gland).
A number of psychiatric syndromes feature depressed mood as a main symptom. The mood disorders are a group of disorders considered to be primary disturbances of mood. These include major depressive disorder (MDD; commonly called major depression or clinical depression) where a person has at least two weeks of depressed mood or a loss of interest or pleasure in nearly all activities; and dysthymia, a state of chronic depressed mood, the symptoms of which do not meet the severity of a major depressive episode. Another mood disorder, bipolar disorder, features one or more episodes of abnormally elevated mood, cognition and energy levels, but may also involve one or more depressive episodes. When the course of depressive episodes follows a seasonal pattern, the disorder (major depressive disorder, bipolar disorder, etc.) may be described as a seasonal affective disorder.
Outside the mood disorders: borderline personality disorder commonly features depressed mood; adjustment disorder with depressed mood is a mood disturbance appearing as a psychological response to an identifiable event or stressor, in which the resulting emotional or behavioral symptoms are significant but do not meet the criteria for a major depressive episode;:355 and posttraumatic stress disorder, an anxiety disorder that sometimes follows trauma, is commonly accompanied by depressed mood.
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Many psychological assessments related to testing for depression and/or for the severity of depressive symptoms exist, assessments such as the Beck Depression Inventory and Children's Depression Inventory test for depression and/or depressive symptoms.
Depressed mood may not require any professional treatment, and may be a normal reaction to certain life events, a symptom of some medical conditions, or a side effect of some drugs or medical treatments. A prolonged depressed mood, especially in combination with other symptoms, may lead to a diagnosis of a psychiatric or medical condition, which may benefit from treatment. Different sub-divisions of depression have different treatment approaches.
Given an accurate diagnosis of major depressive disorder, in general the type of treatment (psychotherapy and/or antidepressants, alternative therapies, or active intervention) is "less important than getting depressed patients involved in an active therapeutic program."
Moderate levels of physical activity can treat depression by increasing the levels of endorphins and the neurotransmitters serotonin, dopamine, and norepinephrine. Exercise allows individuals to improve their health while building new relationships with others and bolstering the sense of community that comes with exercising as a group. Group activities can lower depression by increasing depressed individuals’ ability to interact with others. Exercise also increases individuals’ self-confidence by promoting social skills that people with depression often lack and interrupts the cycle of isolation from the general population that can further increase depression. Exercise fosters non-demanding behaviors while allowing people to socialize and identify themselves as part of the general population.
Women are more prone to depression and this could be explained by gender roles and norms attached to those roles. Women are expected to care for family and friends, but without strong, stable supportive relationships they are more susceptible to depressive symptoms.
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