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, alone, irritable, hurt, or restless. They may lose interest in activities that were once 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, aches, pains, digestive problems or reduced energy may also be present.
Depressed mood is not always a psychiatric disorder. It 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. Depressed mood is also a primary or associated feature of certain psychiatric syndromes such as clinical depression.
Adversity in childhood, such as bereavement, neglect, unequal parental treatment of siblings, physical abuse or sexual abuse, significantly increases the likelihood of experiencing depression over the life course.
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.
Depressed mood can be the result of a number of infectious diseases, nutritional deficiencies, 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 an extremely intense depressive 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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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.
- Salmans, Sandra (1997). Depression: Questions You Have – Answers You Need. People's Medical Society. ISBN 978-1-882606-14-6.
- "NIMH · Depression". nimh.nih.gov. Retrieved 15 October 2012.
- Lindert J, von Ehrenstein OS, Grashow R, Gal G, Braehler E, Weisskopf MG (April 2014). "Sexual and physical abuse in childhood is associated with depression and anxiety over the life course: systematic review and meta-analysis". Int J Public Health 59 (2): 359–72. doi:10.1007/s00038-013-0519-5. PMID 24122075.
- Christine Heim; D. Jeffrey Newport; Tanja Mletzko; Andrew H. Miller; Charles B. Nemeroff. "The link between childhood trauma and depression: Insights from HPA axis studies in humans". Psychoneuroendocrinology 33 (6): 693–710. doi:10.1016/j.psyneuen.2008.03.008. Retrieved 2014-04-20.
- Pillemer, Karl; Suitor, J. Jill; Pardo, Seth; Henderson Jr, Charles (2010). "Mothers' Differentiation and Depressive Symptoms Among Adult Children". Journal of Marriage and Family 72 (2): 333–345. doi:10.1111/j.1741-3737.2010.00703.x. PMC 2894713. PMID 20607119.
- Schmidt, Peter (2005). "Mood, Depression, and Reproductive Hormones in the Menopausal Transition". The American Journal of Medicine. 118 Suppl 12B (12): 54–8. doi:10.1016/j.amjmed.2005.09.033. PMID 16414327.
- Rashid, T.; Heider, I. (2008). "Life Events and Depression". Annals of Punjab Medical College 2 (1). Retrieved 15 October 2012.
- Ehret M, Sobieraj DM (February 2014). "Prevention of interferon-alpha-associated depression with antidepressant medications in patients with hepatitis C virus: a systematic review and meta-analysis". Int. J. Clin. Pract. 68 (2): 255–61. doi:10.1111/ijcp.12268. PMID 24372654.
- Murray ED, Buttner N, Price BH. (2012) Depression and Psychosis in Neurological Practice. In: Neurology in Clinical Practice, 6th Edition. Bradley WG, Daroff RB, Fenichel GM, Jankovic J (eds.) Butterworth Heinemann. April 12, 2012. ISBN 978-1437704341
- Saravane, D; Feve, B; Frances, Y; Corruble, E; Lancon, C; Chanson, P; Maison, P; Terra, JL et al. (2009). "Drawing up guidelines for the attendance of physical health of patients with severe mental illness". L'Encephale 35 (4): 330–9. doi:10.1016/j.encep.2008.10.014. PMID 19748369.
- Rustad, JK; Musselman, DL; Nemeroff, CB (2011). "The relationship of depression and diabetes: Pathophysiological and treatment implications". Psychoneuroendocrinology 36 (9): 1276–86. doi:10.1016/j.psyneuen.2011.03.005. PMID 21474250.
- Li, M; Fitzgerald, P; Rodin, G (2012). "Evidence-based treatment of depression in patients with cancer". Journal of clinical oncology : official journal of the American Society of Clinical Oncology 30 (11): 1187–96. doi:10.1200/JCO.2011.39.7372. PMID 22412144.
- Gabbard, Glen O. Treatment of Psychiatric Disorders 2 (3rd ed.). Washington, DC: American Psychiatric Publishing. p. 1296.
- American Psychiatric Association (2000a). Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision: DSM-IV-TR. Washington, DC: American Psychiatric Publishing, Inc. ISBN 0-89042-025-4.
- Vieweg, W. V.; Fernandez, D. A.; Beatty-Brooks, M; Hettema, J. M.; Pandurangi, A. K.; Pandurangi, Anand K. (May 2006). "Posttraumatic Stress Disorder: Clinical Features, Pathophysiology, and Treatment". Am. J. Med. 119 (5): 383–90. doi:10.1016/j.amjmed.2005.09.027. PMID 16651048.
- Kovacs, M. (1992). Children's Depression Inventory. North Tonawanda, NY: Multi-Health Systems, Inc.
- Cheog J et al. for PsychCentral.com. Last reviewed August 26, 2010. Frequently Asked Questions About Depression Accessed May 11, 2013
- Staff, UK National Institute for Health and Clinical Excellence (NICE) October 2009. 
- Khan, Arif; James Faucett; Pesach Lichtenberg; Irving Kirsch; Walter A. Brown (July 30, 2012). "A Systematic Review of Comparative Efficacy of Treatments and Controls for Depression". PLoS ONE 7 (7): e41778. doi:10.1371/journal.pone.0041778. PMC 3408478. PMID 22860015.
- Craft and Perna 2004….. Craft LL, Perna FM (2004) The beneﬁts of exercise for the clinically depressed. Prim Care Companion J Clin Psychiatry 6:104–111
- Skrinar et al. Fitness: a viable adjunct to treatment for young adults with psychiatric disabilities. Psychosocial Rehabilitation Journal, 1992, vol. 15 issue 3 20-28
- Pelham & Campagna (1993) The effects of exercise therapy on clients in a psychiatric rehabilitation program. Psychosocial rehabilitation Journal. Vol 16(4) 75-84
- Lopresti, AL; Hood SD; Drummond PD (May 15, 2013). "A review of lifestyle factors that contribute to important pathways associated with major depression: diet, sleep and exercise". J Affect Disord 148 (1): 12–27. doi:10.1016/j.jad.2013.01.014. PMID 23415826. Retrieved 9 February 2014.
- Madhav, Goyal; Sonal Singh, Erica M. S. Sibinga, Neda F. Gould, Anastasia Rowland-Seymour, Ritu Sharma, Zackary Berger, Dana Sleicher, David D. Maron, Hasan M. Shihab, Padmini D. Ranasinghe, Shauna Linn, Shonali Saha, Eric B. Bass, Jennifer A. Haythornthwaite (January 6, 2014). "Meditation Programs for Psychological Stress and Well-being". JAMA Intern Med 174 (3): 357–68. doi:10.1001/jamainternmed.2013.13018. PMID 24395196.
- Even, C; Schröder CM; Friedman S; Rouillon F (May 2008). "Efficacy of light therapy in nonseasonal depression: a systematic review.". J Affect Disord 108 (1-2): 11–23. doi:10.1016/j.jad.2007.09.008. PMID 17950467.
- Praschak-Rieder, N; Willeit M; Neumeister A; Hilger E; Kasper S (1999). "Therapeutic sleep deprivation and phototherapy". Wien Med Wochenschr 149 (18): 520–524. PMID 10637957.
- Taylor, Gemma; Ann McNeill; Alan Girling; Amanda Farley; Nicola Lindson-Hawley; Paul Aveyard (February 13, 2014). "Change in mental health after smoking cessation: systematic review and meta-analysis". BMJ 348: g1151. doi:10.1136/bmj.g1151. PMC 3923980. PMID 24524926.