Symptoms show considerable variation but usually include:

An initial state of "DAZE" with some constriction of the field of consciousness and narrowing of attention, inability to comprehend stimuli, disorientation. Followed either by further withdrawal from the surrounding situation to the extent of a dissociative stupor or by agitating and over activity.

The signs are: Tachycardia (increased heart rate), Sweating, Hyperventilation (increased breathing).

The symptoms usually appear within minutes of the impact of the stressful stimulus and disappear within 2–3 days.

The most common stress reactions include:

- The slowing of reaction time

- Slowness of thought

- Difficulty prioritizing tasks

- Difficulty initiating routine tasks

- Preoccupation with minor issues and familiar tasks

- Indecision and lack of concentration

- Loss of initiative with fatigue

- Exhaustion

Combat stress reaction symptoms align with the symptoms also found in psychological trauma, which is closely related to post-traumatic stress disorder (PTSD). CSR differs from PTSD (among other things) in that a PTSD diagnosis requires a duration of symptoms over one month, which CSR does not.

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.

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 mechanisms for a psychological etiology have been proposed, including theories based on misdiagnoses of an underlying mental illness, stress, or classical conditioning. Many people with MCS meet the criteria for major depressive disorder or anxiety disorder. Other proposed explanations include somatoform disorder, panic disorder, migraine, chronic fatigue syndrome, or fibromyalgia, where symptoms such as brain fog and headaches can be triggered by chemicals or inhalants. Through behavioral conditioning, they may develop real, but unintentionally psychologically produced, symptoms such as anticipatory nausea when they encounter certain odors or other perceived triggers. Affected individuals may also have a tendency to "catastrophically misinterpret benign physical symptoms" or simply have a disturbingly acute sense of smell. The personality trait "absorption", in which individuals are predisposed to becoming deeply immersed in sensory experiences, may be stronger in individuals reporting symptoms of MCS. Behaviors exhibited by MCS sufferers may reflect broader sociological fears about industrial pollution and broader societal trends of technophobia and chemophobia.

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.

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.

Fibromyalgia is classed as a disorder of pain processing due to abnormalities in how pain signals are processed in the central nervous system. The American College of Rheumatology classifies fibromyalgia as being a functional somatic syndrome. The expert committee of the European League Against Rheumatism classifies fibromyalgia as a neurobiological disorder and as a result exclusively give pharmacotherapy their highest level of support. The International Classification of Diseases (ICD-10) lists fibromyalgia as a diagnosable disease under "Diseases of the musculoskeletal system and connective tissue," under the code M79-7, and states that fibromyalgia syndrome should be classified as a functional somatic syndrome rather than a mental disorder. Although mental disorders and some physical disorders commonly are co-morbid with fibromyalgia – especially anxiety, depression, irritable bowel syndrome, and chronic fatigue syndrome – the ICD states that these should be diagnosed separately.

Differences in psychological and autonomic nervous system profiles among affected individuals may indicate the existence of fibromyalgia subtypes. A 2007 review divides individuals with fibromyalgia into four groups as well as "mixed types":

1. "extreme sensitivity to pain but no associated psychiatric conditions" (may respond to medications that block the 5-HT3 receptor)

2. "fibromyalgia and comorbid, pain-related depression" (may respond to antidepressants)

3. "depression with concomitant fibromyalgia syndrome" (may respond to antidepressants)

4. "fibromyalgia due to somatization" (may respond to psychotherapy)

Panphobia, omniphobia, pantophobia, or panophobia is a vague and persistent dread of some unknown evil. Panphobia is not registered as a type of phobia in medical references.

The defining symptoms of fibromyalgia are chronic widespread pain, fatigue, sleep disturbance, and heightened pain in response to tactile pressure (allodynia). Other symptoms may include tingling of the skin (paresthesias), prolonged muscle spasms, weakness in the limbs, nerve pain, muscle twitching, palpitations, and functional bowel disturbances.

Many people experience cognitive dysfunction (known as "fibrofog"), which may be characterized by impaired concentration, problems with short and long-term memory, short-term memory consolidation, impaired speed of performance, inability to multi-task, cognitive overload, and diminished attention span. Fibromyalgia is often associated with anxiety and depressive symptoms.

Other symptoms often attributed to fibromyalgia that may be due to a comorbid disorder include myofascial pain syndrome, also referred to as chronic myofascial pain, diffuse non-dermatomal paresthesias, functional bowel disturbances and irritable bowel syndrome, genitourinary symptoms and interstitial cystitis, dermatological disorders, headaches, myoclonic twitches, and symptomatic hypoglycemia. Although fibromyalgia is classified based on the presence of chronic widespread pain, pain may also be localized in areas such as the shoulders, neck, low back, hips, or other areas. Many sufferers also experience varying degrees of myofascial pain and have high rates of comorbid temporomandibular joint dysfunction. 20–30% of people with rheumatoid arthritis and systemic lupus erythematosus may also have fibromyalgia.

Malingering is the fabricating of symptoms of mental or physical disorders for a variety of reasons such as financial compensation (often tied to fraud); avoiding school, work or military service; obtaining drugs; or as a mitigating factor for sentencing in criminal cases. It is not a medical diagnosis. Malingering is typically conceptualized as being distinct from other forms of excessive illness behaviour such as somatization disorder and factitious disorder, e.g., in DSM-5, although not all mental health professionals agree with this formulation.

Failure to detect actual cases of malingering imposes an economic burden on health care systems; workers compensation programs; and disability programs, e.g., Social Security Disability Insurance (United States) and U.S. Department of Veterans Affairs disability benefits. False attribution of malingering often harms genuine patients or claimants.

Shell shock is a phrase coined in World War I to describe the type of posttraumatic stress disorder many soldiers were afflicted with during the war (before PTSD itself was a term).

It is reaction to the intensity of the bombardment and fighting that produced a helplessness appearing variously as panic and being scared, or flight, an inability to reason, sleep, walk or talk.

During the War, the concept of shell shock was ill-defined. Cases of 'shell shock' could be interpreted as either a physical or psychological injury, or simply as a lack of moral fibre. The term "shell shock" is still used by the Veterans Administration to describe certain parts of PTSD but mostly it has entered into popular imagination and memory, and is often identified as the signature injury of the War.

In World War II and thereafter, diagnosis of 'shell shock' was replaced by that of combat stress reaction, a similar but not identical response to the trauma of warfare and bombardment.

There is no specific phobia in the DSM-5 which provides criteria for an all-encompassing fear of everything, though the defining symptom for Generalized Anxiety Disorder in this manual is "excessive anxiety and worry (apprehensive expectation) about a number of events or activities." Another very relatable state of mind is paranoia, in which one fears that unknown threats could, and most likely will, come from anyone, with distrust potentially leading to a loss of touch with reality. Delusional disorder is a more severe form of this type of disorder. Relevant academic literature may point to panphobia as merely a piece of such more complex states of mental disorder. Pseudoneurotic schizophrenia may be diagnosable in patients who, in addition to panphobia, also exhibit symptoms of "pananxiety", "panambivalence", and to a lesser extent, "chaotic sexuality". These persons differ from generalized anxiety sufferers in that they have "free-floating anxiety that rarely subsides" and are clinically diagnosable as having borderline personality disorder in the DSM-IV-TR. No significant changes related to this personality disorder were made in transitioning to the DSM-5, suggesting the diagnostic criteria are still appropriate.

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.

Da Costa's syndrome is generally considered a physical manifestation of an anxiety disorder.

Studies have mixed results as to whether a gradual onset or sudden onset is more frequent.

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.

Although it is listed in the ICD-10 under "somatoform autonomic dysfunction", the term is no longer in common use by any medical agencies and has generally been superseded by more specific diagnoses.

The orthostatic intolerance observed by Da Costa has since also been found in patients diagnosed with chronic fatigue syndrome, postural orthostatic tachycardia syndrome (POTS) and mitral valve prolapse syndrome. In the 21st century, this intolerance is classified as a neurological condition. Exercise intolerance has since been found in many organic diseases.

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.

During the early stages of World War I, soldiers from the British Expeditionary Force began to report medical symptoms after combat, including tinnitus, amnesia, headaches, dizziness, tremors, and hypersensitivity to noise. While these symptoms resembled those that would be expected after a physical wound to the brain, many of those reporting sick showed no signs of head wounds. By December 1914 as many as 10% of British officers and 4% of enlisted men were suffering from "nervous and mental shock".

The term "shell shock" came into use to reflect an assumed link between the symptoms and the effects of explosions from artillery shells. The term was first published in 1915 in an article in "The Lancet" by Charles Myers. Some 60–80% of shell shock cases displayed acute neurasthenia, while 10% displayed what would now be termed symptoms of conversion disorder, including mutism and fugue.

The number of shell shock cases grew during 1915 and 1916 but it remained poorly understood medically and psychologically. Some doctors held the view that it was a result of hidden physical damage to the brain, with the shock waves from bursting shells creating a cerebral lesion that caused the symptoms and could potentially prove fatal. Another explanation was that shell shock resulted from poisoning by the carbon monoxide formed by explosions.

At the same time an alternative view developed describing shell shock as an emotional, rather than a physical, injury. Evidence for this point of view was provided by the fact that an increasing proportion of men suffering shell shock symptoms had not been exposed to artillery fire. Since the symptoms appeared in men who had no proximity to an exploding shell, the physical explanation was clearly unsatisfactory.

In spite of this evidence, the British Army continued to try to differentiate those whose symptoms followed explosive exposure from others. In 1915 the British Army in France was instructed that:

However, it often proved difficult to identify which cases were which, as the information on whether a casualty had been close to a shell explosion or not was rarely provided.

Sundowning, or sundown syndrome, is a neurological phenomenon associated with increased confusion and restlessness in patients with delirium or some form of dementia. Most commonly associated with Alzheimer's disease, but also found in those with other forms of dementia, the term "sundowning" was coined due to the timing of the patient's confusion. For patients with sundowning syndrome, a multitude of behavioral problems begin to occur in the evening or while the sun is setting. Sundowning seems to occur more frequently during the middle stages of Alzheimer's disease and mixed dementia. Patients are generally able to understand that this behavioral pattern is abnormal. Sundowning seems to subside with the progression of a patient's dementia. Research shows that 20–45% of Alzheimer's patients will experience some sort of sundowning confusion.

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.

Lethargy is a state of tiredness, weariness, fatigue, or lack of energy. It can be accompanied by depression, decreased motivation, or apathy. Lethargy can be a normal response to inadequate sleep, overexertion, overworking, stress, lack of exercise, improper nutrition, boredom, or a symptom of a disorder. It may also be a side-effect of medication or caused by an interaction between medications or medication(s) and alcohol. When part of a normal response, lethargy often resolves with rest, adequate sleep, decreased stress, physical exercise and good nutrition.