The terms 'nervous breakdown' and 'mental breakdown' are not formally defined in a diagnostic system such as the DSM-IV or ICD-10. The terms are not often used in the scientific literature on mental illness.
Specific cases are sometimes described as a 'breakdown' only after a person cannot function in day-to-day life. It is a temporary condition. 
Different things can cause a mental breakdown. A study found that problems with intimate relationships, such as divorce or 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, as these accounted for 28% of nervous breakdowns in 1957, 12% in 1976, and only 5.6% in 1996.
In themselves, nervous breakdowns are considered a 'health problem' by most professionals.
- Rapport LJ, Todd RM, Lumley MA, Fisicaro SA. 1998. The diagnostic meaning of "nervous breakdown" among lay populations. J Pers Assess. 71(2):242-52.
- Mayo Clinic mental breakdown
- Hallowell, Edward M & John Ratey. 2005. Delivered from distraction: getting the most out of life with attention deficit disorder. Ballentine Books. ISBN 0-345-44231-8
- Swindle R Jr, Heller K, Pescosolido B, Kikuzawa S. 2000. Responses to nervous breakdowns in America over a 40-year period. Mental health policy implications. Am Psychol. 55 (7) 740-9.