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Mental breakdown (also known as a nervous breakdown) is a colloquial term for an acute, time-limited psychiatric disorder that manifests primarily as severe stress-induced depression, anxiety or dissociation in a previously functional individual, to the extent that they are no longer able to function on a day-to-day basis until the disorder is resolved. A mental breakdown is defined by its temporary nature, and often closely tied to psychological burnout, severe overwork, sleep deprivation and similar stressors, which combine to temporarily overwhelm an individual with otherwise sound mental faculties. A mental breakdown also shares many symptoms with the acute phase of post-traumatic stress disorder.
The terms "nervous breakdown" and "mental breakdown" have not been formally defined through a medical diagnostic system such as the DSM-IV or ICD-10, and are nearly absent from current scientific literature regarding mental illness. Although "nervous breakdown" does not necessarily have a rigorous or static definition, surveys of laypersons suggest that the term refers to a specific acute time-limited reactive disorder, involving symptoms such as anxiety or depression, usually precipitated by external stressors. Many health experts today refer to a nervous breakdown as a “modern mental health crisis.” 
Specific cases are sometimes described as a "breakdown" only after a person becomes unable to function in day-to-day life.
In How Everyone Became Depressed: The Rise and Fall of the Nervous Breakdown (2013), Edward Shorter, a professor of psychiatry and the history of medicine, argues for a return to the old fashioned concept of nervous illness:
Take the women that you know. About half of them are depressed. Or at least that is the diagnosis that they got when they were put on antidepressants. ... They go to work but they are unhappy and uncomfortable; they are somewhat anxious; they are tired; they have various physical pains – and they tend to obsess about the whole business. There is a term for what they have, and it is a good old-fashioned term that has gone out of use. They have nerves or a nervous illness. It is an illness not just of mind or brain, but a disorder of the entire body. ... We have a package here of five symptoms–mild depression, some anxiety, fatigue, somatic pains, and obsessive thinking. ... We have had nervous illness for centuries. When you are too nervous to function ... it is a nervous breakdown. But that term has vanished from medicine, although not from the way we speak. ... The nervous patients of yesteryear are the depressives of today. That is the bad news. ... There is a deeper illness that drives depression and the symptoms of mood. We can call this deeper illness something else, or invent a neologism, but we need to get the discussion off depression and onto this deeper disorder in the brain and body. That is the point.—Edward Shorter, Faculty of Medicine, University of Toronto.
"In eliminating the nervous breakdown, psychiatry has come close to having its own nervous breakdown."
"Nerves stand at the core of common mental illness, no matter how much we try to forget them."—Peter J. Tyrer, FMedSci, Professor of Community Psychiatry, Imperial College, London
Causes of such breakdowns are varied. A 1996 study found that problems with intimate relationships, such as divorce or marital separation, contributed to 24% of nervous breakdowns. Problems at work and school accounted for 17% of cases, and financial problems for 11%. Surveys suggest that in the United States, health problems have decreased in importance as a contributor to nervous breakdowns. Health problems accounted for 28% of nervous breakdowns in 1957, 12% in 1976, and only 5.6% in 1996.
Rapport, Todd, Lumley, and Fisicaro suggest that the closest DSM-IV diagnostic category to nervous breakdown is Adjustment Disorder with Mixed Anxiety and Depressed Mood (Acute). Adjustment disorders and nervous breakdowns are both acute reactions to stress that resolve after removal of the stressor. However, DSM-IV excludes from adjustment disorders cases secondary to bereavement, which contributes to approximately 6–8% of nervous breakdowns.
Nervous breakdowns may share some features of acute stress disorder and post-traumatic stress disorder, in that these each occur in response to an external stressor, and may be marked with sleep disturbance, diminished concentration, and mood lability. However, the symptoms of nervous breakdown do not include the constellation of re-experienced trauma, dissociation, avoidance, and numbing of general responsiveness that are associated with the other two disorders, and the types of stressors linked to a nervous breakdown are generally less extreme.
Nervous breakdowns may share many features of mixed anxiety-depressive disorder (MADD). However, the definition of MADD suggests a chronic condition, in contrast to the acute, short-term nature of a nervous breakdown.