A mental breakdown (also known as a nervous breakdown) is an acute, time-limited mental 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 nervous breakdown is defined by its temporary nature, and often closely tied to psychological burnout, severe overwork, sleep deprivation, and similar stressors, which may combine to temporarily overwhelm an individual with otherwise sound mental functions.

The terms "nervous breakdown" and "mental breakdown" have not been formally defined through a medical diagnostic system such as the DSM-5 or ICD-10, and are nearly absent from current scientific literature regarding mental illness. Although "nervous breakdown" is not rigorously defined, 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 the emotional and physical demands on a person's life are so great as to prevent him or her from performing activities of daily living or, less strictly, only when those demands prevent him/her from performing his/her familial or occupational duties.

Nervous breakdowns are often caused by serious ongoing mental health disorders.

Nearly every day, others may see that the person's activity level is not normal. People suffering from depression may be overly active (psychomotor agitation) or be very lethargic (psychomotor retardation). If a person is agitated, they may find it difficult to sit still, may pace the room, wring their hands, or fidget with clothes or objects. Someone with psychomotor retardation tends to move sluggishly, may move across a room very slowly, avert their eyes, sit slumped in a chair and speak slowly, saying little. They might say that their arms and legs feel heavy.

To meet diagnostic criteria, changes in motor activity must be so abnormal that it can be observed by others. Personal reports of feeling restless or feeling slow do not count towards the diagnostic criteria.

A self-disorder, also called ipseity disturbance, is a psychological phenomenon of disruption or diminishing of a person's sense of minimal (or basic) self. The sense of minimal self refers to the very basic sense of having experiences that are one's own; it has no properties, unlike the more extended sense of self, the narrative self, which is characterized by the person's reflections on themselves as a person, things they like, their identity, and other aspects that are the result of reflection on one's self. Disturbances in the sense of minimal self, as measured by the Examination of Anomalous Self-Experience (EASE), aggregate in the schizophrenia spectrum disorders, to include schizotypal personality disorder, and distinguish them from other conditions such as psychotic bipolar disorder and borderline personality disorder.

Nearly every day, the person may sleep excessively, known as hypersomnia, or not enough, known as insomnia.

Insomnia is the most common type of sleep disturbance for people who are clinically depressed and is often associated with a melancholic type of depression. Symptoms of insomnia include trouble falling asleep, trouble staying asleep, and/or waking up too early in the morning.

Hypersomnia is a less common type of sleep disturbance. It may include sleeping for prolonged periods at night or increased sleeping during the daytime. The sleep may not be restful, and the person may feel sluggish despite many hours of sleep. This impacts their everyday activities and ability to focus at home or work. According to the United States National Library of Medicine, people with seasonal affective disorder (SAD) may sleep longer during the winter months. Hypersomnia is often associated with an atypical depression. Hypersomnia is not as common as insomnia and up to 40% of people exhibit hypersomnia from time to time.

Cognitive symptoms are mainly from deficits in attention, memory, and reaction time. The deficits are in the range of 0.5 to 1.0 standard deviations below expected values, and are likely to affect day-to-day activities. Simple and complex information processing speed, and functions entailing working memory over long time periods were moderately to extensively impaired. These deficits are generally consistent with those reported by patients. Perceptual abilities, motor speed, language, reasoning, and intelligence did not appear to be significantly altered. There is an increased frequency of neuropsychiatric and neuropsychological symptoms in persons with CFS.

The most commonly used diagnostic criteria and definition of CFS for research and clinical purposes were published by the United States Centers for Disease Control and Prevention (CDC). The CDC currently recommends the following criteria for diagnosis:

1. Significantly lowered ability to participate in activities that were routine before the onset of the condition, and persisting more than six months

2. Physical or mental activity causes worsening symptoms that would not have been problematic before the onset of the condition, (post-exertional malaise (PEM))

3. Sleep problems

Additionally, one of the following symptoms must be present:

- Difficulty with thinking and memory

- Worsening of problems with standing or sitting

Other common symptoms may include:

- Muscle pain, joint pain, and headache pain

- Tender lymph nodes in the neck or armpits

- Sore throat

- Irritable bowel syndrome

- Night sweats

- Sensitivities to foods, odors, chemicals, or noise

The CDC proposes that persons with symptoms resembling those of CFS consult a physician to rule out several treatable illnesses: Lyme disease, "sleep disorders, major depressive disorder, alcohol/substance abuse, diabetes, hypothyroidism, mononucleosis (mono), lupus, multiple sclerosis (MS), chronic hepatitis and various malignancies." Medications can also cause side effects that mimic symptoms of CFS. Central sensitization, or increased sensitivity to sensory stimuli such as pain have been observed in CFS. Sensitivity to pain increases post-exertionally, which is opposite to the normal pattern.

Late life depression refers to a major depressive episode occurring for the first time in an older person (usually over 50 or 60 years of age). Concurrent medical problems and lower functional expectations of elderly patients often obscure the degree of impairment. Typically, elderly patients with depression do not report depressed mood, but instead present with less specific symptoms such as insomnia, anorexia, and fatigue. Elderly persons sometimes dismiss less severe depression as an acceptable response to life stress or a normal part of aging.

Patients with psychoorganic syndrome often complain about headaches, dizziness, unsteadiness when walking, poor tolerance to the heat, stuffiness, atmospheric pressure changes, loud sounds, neurological symptoms.

The common reported psychological symptoms include:

- loss of memory and concentration

- emotional liability

- Clinical fatigue

- long term major depression

- severe anxiety

- reduced intellectual ability

The cognitive and behavioral symptoms are chronic and have little response to treatment.

Depending on lesion location, some patients may experience visual complications.

Common symptoms of pain disorder are: negative or distorted cognition, such as feelings of despair or hopelessness; inactivity and passivity, in some cases disability; increased pain, sometimes requiring clinical treatment; sleep disturbance and fatigue; disruption of social relationships; depression and/or anxiety. Acute conditions last less than six months while chronic pain disorder lasts six or more months.

There is no neurological or physiological basis for the pain. Pain is reported as more distressing than it should be if there was a physical explanation. People who suffer from this disorder may begin to abuse medication.

To meet criteria for a major depressive episode, a patient must have 5 of these 9 symptoms nearly every day for at least 2 weeks.

1. Depressed or sad mood

2. Anhedonia (loss of interest in pleasurable activities)

3. Sleep disturbance (increased or decreased sleep)

4. Appetite disturbance (increased or decreased appetite) typically with weight change

5. Energy disturbance (increased or decreased energy/activity level), usually fatigue

6. Poor memory and/or concentration

7. Feelings of guilt or worthlessness

8. Psychomotor retardation or agitation (a change in mental and physical speed perceived by other people)

9. Thoughts of wishing you were dead; suicidal ideation or suicide attempts

Minor depressive disorder is very similar to major depressive disorder in the symptoms present. Generally, a person's mood is affected by thoughts and feelings of being sad or down on themself or by a loss of interest in nearly all activities. People can experience ups and downs in their life everyday where an event, action, stress or many other factors can affect their feelings on that day. However, depression occurs when those feelings of sadness persist for longer than a few weeks.

A person is considered to have minor depressive disorder if they experience 2 to 4 depressive symptoms during a 2-week period. The Diagnostic and Statistical Manual of Mental Disorders lists the major depressive symptoms. Depressed mood most of the day and/or loss of interest or pleasure in normal activities must be experienced by the individual to be considered to have minor depressive disorder. Without either of these two symptoms, the disorder is not classified as minor depressive disorder. Other depressive symptoms include significant weight loss or weight gain without trying to diet (an increase/decrease in appetite can provide clues as well), insomnia or hypersomnia, psychomotor agitation or psychomotor retardation, fatigue or loss of energy, and feelings of worthlessness or excessive guilt.

All of these signs can compound on each other to create the last major symptom group of minor depressive disorder: thoughts of death, suicidal thoughts, plans to commit suicide, or a suicide attempt.

Minor depressive disorder differs from major depressive disorder in the number of symptoms present with 5 or more symptoms necessary for a diagnosis of major depressive disorder. Both disorders require either depressed mood or loss of interest or pleasure in normal activities to be one of the symptoms and the symptoms need to be present for two weeks or longer. Symptoms also must be present for the majority of the length of a day and present for a majority of the days in the two-week period. Diagnosis can only occur if the symptoms cause "clinically significant distress or impairment". Dysthymia consists of the same depressive symptoms, but its main differentiable feature is its longer-lasting nature as compared to minor depressive disorder. Dysthymia was replaced in the DSM-5 by persistent depressive disorder, which combined dysthymia with chronic major depressive disorder.

Symptoms are not limited to but may include:

- Increased general confusion as natural light begins to fade and increased shadows appear.

- Agitation and mood swings. Individuals may become fairly frustrated with their own confusion as well as aggravated by noise. Individuals found yelling and becoming increasingly upset with their caregiver is not uncommon.

- Mental and physical fatigue increase with the setting of the sun. This fatigue can play a role in the individual's irritability.

- Tremors may increase and become uncontrollable.

- An individual may experience an increase in their restlessness while trying to sleep. Restlessness can often lead to pacing and or wandering which can be potentially harmful for an individual in a confused state.

Apathy is a lack of feeling, emotion, interest, and concern. Apathy is a state of indifference, or the suppression of emotions such as concern, excitement, motivation, or passion. An apathetic individual has an absence of interest in or concern about emotional, social, spiritual, philosophical, or physical life and the world.

The apathetic may lack a sense of purpose, worth, or meaning in their life. An apathetic person may also exhibit insensibility or sluggishness. In positive psychology, apathy is described as a result of the individuals feeling they do not possess the level of skill required to confront a challenge (i.e. "flow"). It may also be a result of perceiving no challenge at all (e.g. the challenge is irrelevant to them, or conversely, they have learned helplessness). Apathy may be a sign of more specific mental problems such as schizophrenia or dementia. However, apathy is something that all people face in some capacity. It is a natural response to disappointment, dejection, and stress. As a response, apathy is a way to forget about these negative feelings. This type of common apathy is usually only felt in the short-term and when it becomes a long-term or even lifelong state is when deeper social and psychological issues are most likely present.

Apathy should be distinguished from reduced affect, which refers to reduced emotional expression but not necessarily reduced emotion.

Depression is a state of a low mood and aversion to activity that can affect a person's thoughts, behavior, feelings, and sense of well-being. A depressed mood is a normal temporary reaction to life events such as loss of a loved one. It is also a symptom of some physical diseases and a side effect of some drugs and medical treatments. Depressed mood is also a symptom of some mood disorders such as major depressive disorder or dysthymia.

People with a depressed mood may be notably sad, anxious, or empty; they may also feel notably hopeless, helpless, dejected, or worthless. Other symptoms expressed may include senses of guilt, irritability, or anger. Further feelings expressed by these individuals may include feeling ashamed or an expressed restlessness. These individuals may notably lose interest in activities that they once considered pleasurable to family and friends or otherwise experience either a loss of appetite or overeating. Experiencing problems concentrating, remembering general facts or details, otherwise making decisions or experiencing relationship difficulties may also be notable factors in these individuals' depression and may also lead to their attempting or actually dying by suicide.

Expressed insomnia, excessive sleeping, fatigue, and vocalizing general aches, pains, and digestive problems and a reduced energy may also be present in individuals experiencing depression.

Functional somatic syndrome is a term used to refer to physical symptoms that are poorly explained. It encompass disorders such as chronic fatigue syndrome, fibromyalgia, chronic widespread pain, temporomandibular disorder, irritable bowel syndrome, lower back pain, tension headache, atypical face pain, non-cardiac chest pain, insomnia, palpitation, dyspepsia, and dizziness. General overlap exists between this term, somatization, and somatoform.

Several symptoms are associated with the sopite syndrome. Typical responses include:

- drowsiness

- yawning

- disinclination for work

- lack of social participation

- mood changes

- apathy

- sleep disturbances

- other fatigue-related symptoms

The sopite syndrome is distinguished from other manifestations of motion sickness (i.e. nausea, dizziness, etc.) in that it may occur before other symptoms of motion sickness or in their absence. The sopite syndrome may persist even after an individual has adapted to the other symptoms associated with motion-induced sickness.

The core symptom of depersonalization-derealization disorder is the subjective experience of "unreality in one's self", or detachment from one's surroundings. People who are diagnosed with depersonalization also experience an urge to question and think critically about the nature of reality and existence.

Individuals who experience depersonalization can feel divorced from their own personal physicality by sensing their body sensations, feelings, emotions and behaviors as not belonging to themselves. As such, a recognition of one's self breaks down. Depersonalization can result in very high anxiety levels, which can intensify these perceptions even further.

Individuals with depersonalization describe feeling disconnected from their physicality; feeling as if they are not completely occupying their own body; feeling as if their speech or physical movements are out of their control; feeling detached from their own thoughts or emotions; and experiencing themselves and their lives from a distance. While depersonalization involves detachment from one's "self", individuals with derealization feel detached from their "surroundings", as if the world around them is foggy, dreamlike, or visually distorted. Some people with depersonalization disorder also have visual alterations such as rapid fluctuations in light. While the exact cause of these perceptual changes has not been determined, it is thought that they may be due to previous drug use. These perceptual changes differ from true hallucinatory phenomena, as they are closer to being optical distortions or illusions rather than psychotic breaks from reality. Individuals with the disorder commonly describe a feeling as though time is "passing" them by and they are not in the notion of the present. These experiences which strike at the core of a person's identity and consciousness may cause a person to feel uneasy or anxious.

Factors that tend to diminish symptoms are comforting personal interactions, intense physical or emotional stimulation, and relaxation. Distracting oneself (by engaging in conversation or watching a movie, for example) may also provide temporary relief. Some other factors that are identified as relieving symptom severity are diet and/or exercise, while alcohol and fatigue are listed by some as worsening their symptoms.

First experiences with depersonalization may be frightening, with patients fearing loss of control, dissociation from the rest of society and functional impairment. The majority of people with depersonalization-derealization disorder misinterpret the symptoms, thinking that they are signs of serious psychosis or brain dysfunction. This commonly leads to an increase of anxiety experienced by the patient, and obsession, which contributes to the worsening of symptoms.

Occasional, brief moments of mild depersonalization can be experienced by many members of the general population; however, depersonalization-derealization disorder occurs when these feelings are strong, severe, persistent, or recurrent and when these feelings interfere with daily functioning.

Ethnicities show differences in how they express their discomfort and on how acceptable shows of pain and its tolerance are.

Most obvious in adolescence, females suffer from this disorder more than males, and females reach out more.

More unexplainable pains occur as people get older. Typically, younger children complain of only one symptom, commonly abdominal pains or headaches. The older they get, the more varied the pain location as well as more locations and increasing frequency.

Functional somatic syndrome is characterized by ambiguous, non-specific symptoms that appear in otherwise-healthy populations. Overlap exists in different syndromes, including gastrointestinal issues, pain, fatigue, cognitive difficulties, and sleep difficulties. Some have proposed to group symptoms into clusters or into one general functional somatic disorder given the finding of correlations between symptoms and underlying etiologies.

The effects of the sopite syndrome may range from minor physical inconveniences to hazardous conditions. Persons who operate automobiles, airplanes, et cetera, may experience impaired motor function due to the motions of the vehicle. These impairments often result in a decreased attention span; persons who consider themselves well-rested may still succumb to drowsiness at inopportune moments. The sopite syndrome may therefore contribute to motor vehicle accidents in which automobile operators fall asleep at the wheel. However, the sopite syndrome itself does not directly result in death.

A subject experiencing the sopite syndrome on a frequent basis may increase the number of hours spent sleeping by fifty percent. A study of motion sickness occurrences in workers on an offshore oil vessel showed a large majority of participants experienced mild symptoms of fatigue. Many participants also experienced severe sleep disturbances. These symptoms were associated with impaired task performance.

ADHD-PI is an attention-concentration deficit that has everything in common with other forms of ADHD except that it has fewer hyperactivity or impulsivity symptoms and has more directed attention fatigue symptoms.

Different countries have used different ways of diagnosing ADHD-PI. In the United Kingdom, diagnosis is based on quite a narrow set of symptoms, and about 0.5–1% of children are thought to have attention or hyperactivity problems.

Attention deficit hyperactivity disorder predominantly inattentive (ADHD-PI or ADHD-I), is one of the three presentations of attention deficit hyperactivity disorder (ADHD). It was formerly known as attention deficit disorder (without hyperactivity), but the term was formally changed in 1994 in the "Diagnostic and Statistical Manual of Mental Disorders", fourth edition ("DSM-IV"), to "ADHD predominantly inattentive" (ADHD-PI).

The 'predominantly inattentive subtype' is similar to the other presentations of ADHD except that it is characterized primarily by problems with inattention or a deficit of sustained attention, such as procrastination, hesitation, and forgetfulness. It differs in having fewer or no typical symptoms of hyperactivity or impulsiveness. Lethargy and fatigue are sometimes reported, but ADHD-PI is a separate condition from the proposed cluster of symptoms known as sluggish cognitive tempo (SCT).

The symptoms most commonly feigned include those associated with mild head injury, fibromyalgia, chronic fatigue syndrome, and chronic pain. Generally, malingerers complain of psychological disorders such as anxiety. Malingering may take the form of dishonest complaints of chronic whiplash pain from automobile accidents. The psychological symptoms experienced by survivors of disaster (post-traumatic stress disorder) are also faked by malingerers.

Individuals use a variety of methods to feign symptoms of illness. Some of these include harming oneself, trying to convince medical professionals one has a disease after learning about its details (such as symptoms) in medical textbooks, taking drugs that provoke certain symptoms common in some diseases, performing excess exercise to induce muscle strain or other physical types of ailments, and overdosing on drugs.

Symptoms range in severity from mild to disabling.

Symptoms are common, but vague and non-specific for the condition. The most common are feeling tired, "brain fog" (short-term memory problems, difficulty concentrating), gastrointestinal problems, headaches, and muscle pain.

A partial list of other symptoms patients have attributed to MCS include: difficulty breathing, pains in the throat, chest, or abdominal region, skin irritation, headaches, neurological symptoms (nerve pain, pins and needles feelings, weakness, trembling, restless leg syndrome), tendonitis, seizures, visual disturbances (blurring, halo effect, inability to focus), anxiety, panic and/or anger, sleep disturbance, suppression of immune system, digestive difficulties, nausea, indigestion/heartburn, vomiting, diarrhea, joint pains, vertigo/dizziness, abnormally acute sense of smell (hyperosmia), sensitivity to natural plant fragrance or natural pine terpenes, dry mouth, dry eyes, and an overactive bladder.